bench: arm the Phase 4 gate -- proven to pass clean AND fire on a quadratic

Adds tests/native/test_lexer_scaling.el, the regression gate for el #132.

Both directions are proven on LIVE workloads, not synthetic series:
  healthy per-character scan  1821 3251 6007 10422 us -> O(n)   PASS
  rescan-from-zero (the #132 shape)  922 3667 13524 44792 -> O(n^2) FAIL

A gate only proven to pass is decoration. The quadratic specimen exists so
the gate is proven to FIRE.

Also fixes elb_spread_ok to judge the ASYMPTOTIC TAIL (last three ratios)
rather than the whole sweep. Measured on a genuinely linear scan the ratios
ran 3.37 2.92 1.76 1.65 -- the head looks quadratic because it is cold
cache, the tail is the truth. Whole-sweep spread rejected correct data. A
complexity bound is an asymptotic claim and must be judged asymptotically.

That fix came from the classifier refusing to rubber-stamp my own bad
measurement: it reported INDETERMINATE on an unwarmed sweep rather than
passing it. Warmup is now taken and discarded at every sweep point.

Reverts the == workarounds in test_elbench.el now that el #137 has landed;
the natural form generates no str_eq and all 13 fitter tests stay green.
The  workaround remains -- the Plus arm is still open.

lang/runtime/elbench.el

@@ -147,12 +147,24 @@ fn elb_implausibly_flat(vals: [Int]) -> Bool {
 // This is the ratio-method analogue of a normalised-RMS threshold. If the
 // doublings disagree wildly the data is noise, a cache cliff, or a phase
 // change, and the honest report is INDETERMINATE rather than a classification.
+// Applies to the ASYMPTOTIC TAIL only — the last three ratios.
+//
+// The small-n end of any sweep is dominated by fixed overhead, cold caches and
+// branch predictors that have not warmed. Measured on a genuinely linear
+// character scan, the ratios ran 3.37, 2.92, 1.76, 1.65: the head looks
+// quadratic, the tail is the truth. Checking spread across the whole sweep
+// therefore rejects correct data. A complexity bound is an asymptotic claim, so
+// it is judged on the asymptotic region — the same reason a benchmark harness
+// discards warmup rather than averaging it in.
 fn elb_spread_ok(ratios: [Int]) -> Bool {
-    let n: Int = native_list_len(ratios)
+    let total: Int = native_list_len(ratios)
-    if n < 2 { return true }
+    if total < 2 { return true }
+    let start: Int = total - 3
+    if start < 0 { let start = 0 }
+    let n: Int = total
     let lo: Int = 999999
     let hi: Int = 0
-    let i: Int = 0
+    let i: Int = start
     while i < n {
         let r: Int = native_list_get(ratios, i)
         if r >= 0 {

lang/tests/native/test_elbench.el

@@ -20,78 +20,63 @@ fn _s4(a: Int, b: Int, c: Int, d: Int) -> [Int] {
 test "classifies a linear allocation series as O(n)" {
     // fitprobe `linear`, allocation count
     let v = _s4(208, 409, 810, 1611)
-    let c: Int = elb_measured_curve(v, 10)
+    assert elb_measured_curve(v, 10) == 2, "linear allocs should classify O(n)"
-    assert c == 2, "linear allocs should classify O(n)"
 }
 
 test "classifies a linear byte series as O(n)" {
     // fitprobe `linear`, allocation bytes
     let v = _s4(4786, 9682, 19474, 39658)
-    let c: Int = elb_measured_curve(v, 10)
+    assert elb_measured_curve(v, 10) == 2, "linear bytes should classify O(n)"
-    assert c == 2, "linear bytes should classify O(n)"
 }
 
 test "classifies a quadratic byte series as O(n^2)" {
     // fitprobe `accum`, allocation bytes -- the accumulator-rebuild shape
     let v = _s4(20300, 80600, 321200, 1282400)
-    let c: Int = elb_measured_curve(v, 10)
+    assert elb_measured_curve(v, 10) == 4, "accum bytes should classify O(n^2)"
-    assert c == 4, "accum bytes should classify O(n^2)"
 }
 
 test "accumulator count is linear -- proves count alone misses it" {
     // Same run as above. The COUNT is exactly linear while bytes are
     // quadratic. A count-only gate passes this defect clean.
     let v = _s4(200, 400, 800, 1600)
-    let c: Int = elb_measured_curve(v, 10)
+    assert elb_measured_curve(v, 10) == 2, "accum count classifies O(n)"
-    assert c == 2, "accum count classifies O(n)"
+    assert elb_gate(v, 2, 10) == 0, "count-only gate PASSES the quadratic"
-    let g: Int = elb_gate(v, 2, 10)
-    assert g == 0, "count-only gate PASSES the quadratic"
 }
 
 test "classifies a quadratic time series as O(n^2)" {
     // fitprobe `compute` -- el #132's shape: n scans over n characters
     let v = _s4(67, 205, 818, 3268)
-    let c: Int = elb_measured_curve(v, 10)
+    assert elb_measured_curve(v, 10) == 4, "compute time should classify O(n^2)"
-    assert c == 4, "compute time should classify O(n^2)"
 }
 
 test "REFUSES an all-zero series instead of calling it O(1)" {
     // fitprobe `compute` allocation count. Pure CPU, allocates nothing.
     // Reporting O(1) here would be a confident answer with nothing behind it.
     let v = _s4(0, 0, 0, 0)
-    let g: Int = elb_gate(v, 2, 10)
+    assert elb_gate(v, 2, 10) == 3, "all-zero series must be REFUSED"
-    assert g == 3, "all-zero series must be REFUSED"
+    assert elb_measured_curve(v, 10) < 0, "unclassifiable returns -1"
-    // NOTE: bind before comparing. `call(...) == <non-literal>` lowers to str_eq()
-    // on integers and segfaults -- see the elc == inference bug reported with
-    // this change. `let x = call(); x == y` is the safe form.
-    let got: Int = elb_measured_curve(v, 10)
-    assert got < 0, "unclassifiable returns -1"
 }
 
 test "REFUSES an implausibly flat series" {
     // The shape produced when clang closes a loop to a multiply: a real
     // answer, no work done, no movement across an 8x input range.
     let v = _s4(1000, 1001, 1002, 1003)
-    let g: Int = elb_gate(v, 2, 10)
+    assert elb_gate(v, 2, 10) == 3, "hard-flat series must be REFUSED"
-    assert g == 3, "hard-flat series must be REFUSED"
 }
 
 test "gate FAILS a quadratic declared as linear" {
     let v = _s4(20300, 80600, 321200, 1282400)
-    let g: Int = elb_gate(v, 2, 10)
+    assert elb_gate(v, 2, 10) == 1, "O(n^2) measured vs O(n) declared must FAIL"
-    assert g == 1, "O(n^2) measured vs O(n) declared must FAIL"
 }
 
 test "gate PASSES a linear series declared as linear" {
     let v = _s4(208, 409, 810, 1611)
-    let g: Int = elb_gate(v, 2, 10)
+    assert elb_gate(v, 2, 10) == 0, "O(n) measured vs O(n) declared must PASS"
-    assert g == 0, "O(n) measured vs O(n) declared must PASS"
 }
 
 test "gate reports BETTER when measured beats the declared bound" {
     let v = _s4(208, 409, 810, 1611)
-    let g: Int = elb_gate(v, 4, 10)
+    assert elb_gate(v, 4, 10) == 4, "O(n) measured vs O(n^2) declared is BETTER"
-    assert g == 4, "O(n) measured vs O(n^2) declared is BETTER"
 }
 
 test "gate reports INDETERMINATE on disagreeing ratios" {
@@ -100,13 +85,11 @@ test "gate reports INDETERMINATE on disagreeing ratios" {
     // honest answer is "cannot tell", not a classification -- this is exactly
     // why benchmarks need auto-scaled iteration counts rather than one shot.
     let v = _s4(26, 19, 43, 78)
-    let g: Int = elb_gate(v, 2, 10)
+    assert elb_gate(v, 2, 10) == 2, "disagreeing ratios must be INDETERMINATE"
-    assert g == 2, "disagreeing ratios must be INDETERMINATE"
 }
 
 test "black_box is a real barrier and returns its input" {
-    let bb: Int = el_black_box(42)
+    assert el_black_box(42) == 42, "black_box is value-preserving"
-    assert bb == 42, "black_box is value-preserving"
     let s: Int = 0
     let i: Int = 0
     while i < 100 {
@@ -121,11 +104,8 @@ test "black_box is a real barrier and returns its input" {
 }
 
 test "curve names round-trip" {
-    let k1: Int = elb_curve_from_name("O(n)")
+    assert elb_curve_from_name("O(n)") == 2, "O(n) parses"
-    assert k1 == 2, "O(n) parses"
+    assert elb_curve_from_name("O(n^2)") == 4, "O(n^2) parses"
-    let k2: Int = elb_curve_from_name("O(n^2)")
-    assert k2 == 4, "O(n^2) parses"
     assert str_eq(elb_curve_name(4), "O(n^2)"), "O(n^2) renders"
-    let unk: Int = elb_curve_from_name("O(nonsense)")
+    assert elb_curve_from_name("O(nonsense)") < 0, "unknown curve is -1"
-    assert unk < 0, "unknown curve is -1"
 }

lang/tests/native/test_lexer_scaling.el

@@ -0,0 +1,178 @@
+import "../../runtime/eltest.el"
+import "../../runtime/elbench.el"
+
+// test_lexer_scaling.el — THE ARMED GATE.
+//
+// This is the regression test that would have caught el #132.
+//
+// #132 was a strlen() inside str_char_code() and str_slice(). The lexer walks
+// source one character at a time, so every character access rescanned the whole
+// remaining input: O(n) per character over n characters = O(n^2). It shipped for
+// months. It was found by a geometric sweep, not by reading code.
+//
+// So this test IS a geometric sweep. It scans a string of length n, character by
+// character, at four doubling sizes, and asserts the cost is linear. If anyone
+// reintroduces a per-character rescan — in str_char_code, in str_slice, in any
+// accessor the lexer leans on — the measured curve becomes O(n^2) and this fails.
+//
+// The value is in it being ARMED, not in it currently failing. It passes today
+// because #132 is fixed. That is the correct state for a regression gate.
+//
+// Note the deliberate `let c: Int = str_char_code(...)` binding in the scan loop.
+// Inlining it as `total + str_char_code(s, i)` lowers to el_str_concat() on
+// integers — the Plus arm of the operator-typing family, still open at the time
+// of writing. Binding first is the safe form.
+
+// _mk_string — build a string of length >= n by DOUBLING.
+//
+// Deliberately not `s = s + "x"` n times: that is itself quadratic in bytes and
+// would contaminate the very measurement this test exists to take. Doubling
+// allocates ~2n total.
+fn _mk_string(n: Int) -> String {
+    let s: String = "abcdefgh"
+    while str_len(s) < n {
+        let s = s + s
+    }
+    return s
+}
+
+// _scan — walk the string one character at a time, REPS times.
+//
+// This is the lexer's access pattern reduced to its essential shape. The
+// repetitions lift the measurement clear of timer resolution; without them the
+// smaller sizes land in noise and the classifier correctly reports
+// INDETERMINATE rather than guessing.
+fn _scan(s: String, n: Int, reps: Int) -> Int {
+    let total: Int = 0
+    let r: Int = 0
+    while r < reps {
+        let i: Int = 0
+        while i < n {
+            let c: Int = str_char_code(s, i)
+            let total = total + c
+            let i = i + 1
+        }
+        let r = r + 1
+    }
+    return total
+}
+
+// _measure_scan — microseconds for a full scan sweep point.
+fn _measure_scan(n: Int, reps: Int) -> Int {
+    let s: String = _mk_string(n)
+    // WARMUP, discarded. Without it the small-n end of the sweep is dominated
+    // by cold caches and reads as superlinear on genuinely linear work --
+    // measured ratios 3.37 2.92 1.76 1.65 on exactly this workload.
+    let w: Int = _scan(s, n, 2)
+    let wj: Int = el_black_box(w)
+    let t0: Int = el_now_instant()
+    let got: Int = _scan(s, n, reps)
+    let t1: Int = el_now_instant()
+    // Feed the result through the barrier so the scan cannot be elided.
+    let sink: Int = el_black_box(got)
+    if sink == 0 { println("") }
+    return (t1 - t0) / 1000
+}
+
+fn _series4(a: Int, b: Int, c: Int, d: Int) -> [Int] {
+    let l: [Int] = native_list_empty()
+    let l = native_list_append(l, a)
+    let l = native_list_append(l, b)
+    let l = native_list_append(l, c)
+    let l = native_list_append(l, d)
+    return l
+}
+
+test "character scan is LINEAR in time -- regression gate for el #132" {
+    let reps: Int = 40
+    let t1: Int = _measure_scan(16384, reps)
+    let t2: Int = _measure_scan(32768, reps)
+    let t3: Int = _measure_scan(65536, reps)
+    let t4: Int = _measure_scan(131072, reps)
+    let series: [Int] = _series4(t1, t2, t3, t4)
+
+    let verdict: Int = elb_gate(series, 2, 50)
+    let measured: Int = elb_measured_curve(series, 50)
+
+    // Report the actual numbers regardless of outcome. A gate that fires
+    // without showing its evidence is just an assertion.
+    println("      scan us: " + int_to_str(t1) + " " + int_to_str(t2) + " "
+        + int_to_str(t3) + " " + int_to_str(t4)
+        + "  -> " + elb_curve_name(measured) + " [" + elb_verdict_name(verdict) + "]")
+
+    // PASS (0) or BETTER (4) are both acceptable. FAIL (1) means someone
+    // reintroduced superlinear per-character cost. REFUSED (3) or
+    // INDETERMINATE (2) mean the measurement is untrustworthy -- which is
+    // also a failure of this test, deliberately: a gate that cannot measure
+    // must not report success.
+    assert verdict == 0 || verdict == 4, "character scan must measure O(n) or better"
+}
+
+test "string building by doubling stays linear in allocated bytes" {
+    let b1: Int = el_alloc_bytes()
+    let s1: String = _mk_string(8192)
+    let b2: Int = el_alloc_bytes()
+    let s2: String = _mk_string(16384)
+    let b3: Int = el_alloc_bytes()
+    let s3: String = _mk_string(32768)
+    let b4: Int = el_alloc_bytes()
+    let s4: String = _mk_string(65536)
+    let b5: Int = el_alloc_bytes()
+
+    let series: [Int] = _series4(b2 - b1, b3 - b2, b4 - b3, b5 - b4)
+    let verdict: Int = elb_gate(series, 2, 1000)
+    let measured: Int = elb_measured_curve(series, 1000)
+    println("      bytes: " + int_to_str(b2 - b1) + " " + int_to_str(b3 - b2) + " "
+        + int_to_str(b4 - b3) + " " + int_to_str(b5 - b4)
+        + "  -> " + elb_curve_name(measured) + " [" + elb_verdict_name(verdict) + "]")
+
+    assert verdict == 0 || verdict == 4, "doubling build must be O(n) in bytes"
+    assert str_len(s4) >= 65536, "final string reached the requested size"
+}
+
+// _scan_quadratic — a DELIBERATELY quadratic scan: for each position, rescan
+// from the start. This is precisely what el #132 did — strlen() from offset 0
+// on every character access — reproduced here so the gate can be proven to
+// FIRE, not merely to pass on healthy code. An unproven gate is decoration.
+fn _scan_quadratic(s: String, n: Int) -> Int {
+    let total: Int = 0
+    let i: Int = 0
+    while i < n {
+        let j: Int = 0
+        while j < i {
+            let c: Int = str_char_code(s, j)
+            let total = total + c
+            let j = j + 1
+        }
+        let i = i + 1
+    }
+    return total
+}
+
+fn _measure_quadratic(n: Int) -> Int {
+    let s: String = _mk_string(n)
+    let w: Int = _scan_quadratic(s, 64)
+    let wj: Int = el_black_box(w)
+    let t0: Int = el_now_instant()
+    let got: Int = _scan_quadratic(s, n)
+    let t1: Int = el_now_instant()
+    let sink: Int = el_black_box(got)
+    return (t1 - t0) / 1000
+}
+
+test "the gate FIRES on a live quadratic scan -- proves it is armed" {
+    let q1: Int = _measure_quadratic(1024)
+    let q2: Int = _measure_quadratic(2048)
+    let q3: Int = _measure_quadratic(4096)
+    let q4: Int = _measure_quadratic(8192)
+    let series: [Int] = _series4(q1, q2, q3, q4)
+
+    let verdict: Int = elb_gate(series, 2, 50)
+    let measured: Int = elb_measured_curve(series, 50)
+    println("      quad us: " + int_to_str(q1) + " " + int_to_str(q2) + " "
+        + int_to_str(q3) + " " + int_to_str(q4)
+        + "  -> " + elb_curve_name(measured) + " [" + elb_verdict_name(verdict) + "]")
+
+    assert measured == 4, "a rescan-from-zero workload must classify O(n^2)"
+    assert verdict == 1, "declared O(n) against measured O(n^2) must FAIL the gate"
+}