add text-processing primitives to el runtime

24 new functions covering counting (str_count, str_count_chars,
str_count_bytes, str_count_lines, str_count_words, str_count_letters,
str_count_digits), finding (str_index_of_all, str_last_index_of,
str_find_chars), transforming (str_repeat, str_reverse,
str_strip_prefix/suffix/chars, str_lstrip, str_rstrip), character
classification (is_letter, is_digit, is_alphanumeric, is_whitespace,
is_punctuation, is_uppercase, is_lowercase), and splitting/joining
(str_split_lines, str_split_chars, str_split_n, str_join).

Phase 1 is byte-level + ASCII character classes. Unicode-grapheme
awareness, normalization, and regex are Phase 2 (filed separately).

Lexer-internal helpers is_digit, is_alpha, is_whitespace renamed to
lex_is_digit, lex_is_alpha, lex_is_whitespace to free the public names
for the runtime exports. The El compiler's lexer.el and the bundled
elc-combined.el both updated.

Codegen registrations: builtin_arity entries for all 24 functions,
is_int_call entries for the Int-returning ones (str_count*,
str_last_index_of, str_find_chars) so the + operator dispatches as
arithmetic when applicable.

Tests: tests/text/ corpus with 8 acceptance cases covering the surface
(count-substring, count-overlap-skip, count-lines-words-letters,
index-of-all, transform-suite, char-classes, split-lines, join). All
pass against a fold-fn-main-aware elc bootstrap (see ELC env var
override in run.sh).

Self-host fixed point: elc-combined.el's emit-main pass does not
currently fold the fn main body into C's main, a pre-existing
condition that surfaces as a 39-line gen2/gen3 diff with empty main
in gen3. The committed dist/platform/elc binary has the fold logic
so all tests pass against it. Filing the elc-combined fold-fn-main
fix separately. This commit does not introduce new self-host drift.

tests/text/examples/char-classes.el

@@ -0,0 +1,19 @@
+// char-classes.el — ASCII character classification.
+// is_letter("A") && is_digit("7") && is_whitespace(" ")
+// && !is_letter("3") && !is_digit("X")
+fn run_test() -> Bool {
+    if !is_letter("A") { return false }
+    if !is_digit("7") { return false }
+    if !is_whitespace(" ") { return false }
+    if is_letter("3") { return false }
+    if is_digit("X") { return false }
+    return true
+}
+
+fn main() -> Void {
+    if run_test() {
+        println("true")
+    } else {
+        println("false")
+    }
+}

tests/text/examples/count-lines-words-letters.el

@@ -0,0 +1,16 @@
+// count-lines-words-letters.el — composite count test.
+// Input "Hello world\nGoodbye world\n":
+//   lines:   2 (each \n closes a line)
+//   words:   4 (Hello, world, Goodbye, world)
+//   letters: 22 (Hello=5 + world=5 + Goodbye=7 + world=5)
+fn run_test() -> String {
+    let s: String = "Hello world\nGoodbye world\n"
+    let lines: Int = str_count_lines(s)
+    let words: Int = str_count_words(s)
+    let letters: Int = str_count_letters(s)
+    return int_to_str(lines) + "/" + int_to_str(words) + "/" + int_to_str(letters)
+}
+
+fn main() -> Void {
+    println(run_test())
+}

tests/text/examples/count-overlap-skip.el

@@ -0,0 +1,11 @@
+// count-overlap-skip.el — non-overlapping advance: "aaaa" / "aa" -> 2.
+// After each match, the cursor advances by len(sub), so overlapping matches
+// are skipped. This is the universal default in C/Python/Go/etc.
+fn run_test() -> Int {
+    let s: String = "aaaa"
+    return str_count(s, "aa")
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}

tests/text/examples/count-substring.el

@@ -0,0 +1,9 @@
+// count-substring.el — non-overlapping substring count.
+fn run_test() -> Int {
+    let s: String = "abc abc abc"
+    return str_count(s, "abc")
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}

tests/text/examples/index-of-all.el

@@ -0,0 +1,21 @@
+// index-of-all.el — every byte offset of a substring.
+// "abXcdXefX" / "X" -> [2, 5, 8]. Empty list when no matches.
+fn run_test() -> String {
+    let positions: [Int] = str_index_of_all("abXcdXefX", "X")
+    let n: Int = native_list_len(positions)
+    let out: String = ""
+    let i: Int = 0
+    while i < n {
+        let p: Int = native_list_get(positions, i)
+        if i > 0 {
+            let out = out + ","
+        }
+        let out = out + int_to_str(p)
+        let i = i + 1
+    }
+    return out
+}
+
+fn main() -> Void {
+    println(run_test())
+}

tests/text/examples/join.el

@@ -0,0 +1,12 @@
+// join.el — list-of-string join with separator.
+fn run_test() -> String {
+    let parts: [String] = native_list_empty()
+    let parts = native_list_append(parts, "alpha")
+    let parts = native_list_append(parts, "beta")
+    let parts = native_list_append(parts, "gamma")
+    return str_join(parts, ", ")
+}
+
+fn main() -> Void {
+    println(run_test())
+}

tests/text/examples/split-lines.el

@@ -0,0 +1,10 @@
+// split-lines.el — \n-delimited split, \r\n folded, trailing empty dropped.
+// "alpha\nbeta\r\ngamma\n" -> ["alpha", "beta", "gamma"], len = 3.
+fn run_test() -> Int {
+    let lines: [String] = str_split_lines("alpha\nbeta\r\ngamma\n")
+    return native_list_len(lines)
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}

tests/text/examples/transform-suite.el

@@ -0,0 +1,13 @@
+// transform-suite.el — repeat, reverse, strip prefix/suffix/chars.
+fn run_test() -> String {
+    let a: String = str_repeat("ab", 3)                      // "ababab"
+    let b: String = str_reverse("hello")                     // "olleh"
+    let c: String = str_strip_prefix("foobar", "foo")        // "bar"
+    let d: String = str_strip_suffix("hello.md", ".md")      // "hello"
+    let e: String = str_strip_chars(" \thello \n", " \t\n")  // "hello"
+    return a + "|" + b + "|" + c + "|" + d + "|" + e
+}
+
+fn main() -> Void {
+    println(run_test())
+}