bench: real black_box barrier + three-signal growth-curve gate

Adds el_black_box (inline asm, +r constraint, memory clobber) and
runtime/elbench.el: a growth-curve classifier that gates time AND
allocation-count AND allocation-bytes, failing if any exceeds its
declared curve.

Refusal is a first-class verdict. The classifier REFUSES rather than
classifying when the largest measurement is below the floor, or when a
series is hard-flat across an 8x input range -- the shape produced when
the optimiser deletes the work. Reporting O(1) there would be a
confident answer with nothing behind it. Disagreeing ratios report
INDETERMINATE rather than a guess.

Deviation from DESIGN.md 6.2, stated in the source: uses consecutive
ratios on a mandated geometric sweep rather than least-squares over
candidate curves. Ratios are directly interpretable on a doubling sweep
and need no floating point; the cost is weaker O(n) vs O(n log n)
separation, reported as an ambiguous band rather than guessed.

Documents the counter scope limit: engram_*.c and libcurl malloc are
NOT tracked, so a flat curve over engram/HTTP-dominated work is not
evidence of anything.

13 tests prove the classifier against real measured series from
fitprobe.el -- including that an accumulator's allocation COUNT is
linear while its bytes are quadratic, and that el #132's pure-CPU shape
reads FLAT on both allocation signals and is caught only by time.

lang/el-compiler/src/codegen.el

@@ -2887,6 +2887,7 @@ fn builtin_arity(name: String) -> Int {
     if str_eq(name, "el_alloc_count") { return 0 }
     if str_eq(name, "el_alloc_bytes") { return 0 }
     if str_eq(name, "el_peak_rss") { return 0 }
+    if str_eq(name, "el_black_box") { return 1 }
     if str_eq(name, "engram_neighbors_json") { return 3 }
     if str_eq(name, "engram_activate_json") { return 2 }
     if str_eq(name, "engram_stats_json") { return 0 }