runtime: restore the three builtins that made elc unrebuildable

The committed elc binary could not be refreshed from its own source. Rebuilding
failed with three implicit-declaration errors: el_mem_check, stdout_to_file,
stdout_restore. The compiler's own source calls all three (compiler.el:472,479,574
and codegen.el:4248) and two are registered in codegen.el's builtin_arity table —
but none were defined in this runtime.

They were found intact in ui/examples/native-hello-ios/NativeHello/el_runtime.c,
a divergent private copy of this runtime that still carried them. Ported verbatim.

Consequence of them being missing: the canonical elc binary was frozen. Source
gained @route dispatch codegen (emit_route_dispatch, codegen.el:3948) and the
@manager boundary-beat seam, but no rebuilt binary could carry them, so
neuron's soul — whose routes.el now calls the compiler-synthesized
el_route_dispatch — could not be built at all.

Verified after the fix:
  - elc rebuilds from current source, clean.
  - Self-hosting fixpoint byte-identical (stage3 == stage2).
  - The rebuilt elc emits el_route_dispatch (2 occurrences in the soul amalgam,
    previously 0) and injects engram_boundary_beat at @manager boundaries,
    i.e. the decorator seam is live rather than inert.

el_mem_check is itself the compiler's memory guard (ELC_MAX_MEM_MB, default
512MB, self-terminates before the OS OOM-killer fires) — so the runtime was
missing the very guard that would have surfaced the compiler's memory blowup
as a clean error instead of a 27GB host-killer.

lang/runtime/el_runtime.c

@@ -18204,3 +18204,70 @@ el_val_t __http_do(el_val_t m, el_val_t u, el_val_t b, el_val_t h, el_val_t t) {
 el_val_t __http_do_map(el_val_t m, el_val_t u, el_val_t b, el_val_t h, el_val_t t) { (void)m; (void)u; (void)b; (void)h; (void)t; return _no_curl_err(); }
 el_val_t __http_do_map_to_file(el_val_t m, el_val_t u, el_val_t b, el_val_t h, el_val_t p) { (void)m; (void)u; (void)b; (void)h; (void)p; return _no_curl_err(); }
 #endif /* !HAVE_CURL */
+
+/* ── Compiler-support builtins ───────────────────────────────────────────────
+ * stdout_to_file / stdout_restore / el_mem_check are called by the El compiler's
+ * own source (compiler.el:472,479,574 and codegen.el:4248) and are registered in
+ * codegen.el's builtin_arity table, but were missing from this runtime — so
+ * rebuilding elc from source failed with three implicit-declaration errors and
+ * the committed elc binary could never be refreshed. The definitions below are
+ * ported verbatim from ui/examples/native-hello-ios/NativeHello/el_runtime.c,
+ * a divergent private copy of this runtime that still carried them.
+ * ──────────────────────────────────────────────────────────────────────────── */
+
+#include <sys/resource.h>
+
+static int _el_saved_stdout_fd = -1;
+
+/* Redirect process stdout to a file; used by the compiler's JS post-processing
+ * pipeline to capture codegen output before piping it onward. */
+el_val_t stdout_to_file(el_val_t pathv) {
+    const char* path = EL_CSTR(pathv);
+    if (!path) return (el_val_t)(int64_t)-1;
+    fflush(stdout);
+    _el_saved_stdout_fd = dup(STDOUT_FILENO);
+    int fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0600);
+    if (fd < 0) return (el_val_t)(int64_t)-1;
+    dup2(fd, STDOUT_FILENO);
+    close(fd);
+    return (el_val_t)(int64_t)0;
+}
+
+el_val_t stdout_restore(void) {
+    if (_el_saved_stdout_fd >= 0) {
+        fflush(stdout);
+        dup2(_el_saved_stdout_fd, STDOUT_FILENO);
+        close(_el_saved_stdout_fd);
+        _el_saved_stdout_fd = -1;
+    }
+    return (el_val_t)(int64_t)0;
+}
+
+/* el_mem_check — self-terminating memory guard for long-running compiler runs.
+ * Called periodically by the compiler to catch runaway growth before the OS
+ * OOM-killer fires. Limit comes from ELC_MAX_MEM_MB (default 512 MB).
+ * macOS reports ru_maxrss in bytes, Linux in kilobytes; normalised to MB. */
+el_val_t el_mem_check(void) {
+    long limit_mb = 512;
+    const char* env_val = getenv("ELC_MAX_MEM_MB");
+    if (env_val && *env_val) {
+        long v = atol(env_val);
+        if (v > 0) limit_mb = v;
+    }
+
+    struct rusage ru;
+    if (getrusage(RUSAGE_SELF, &ru) != 0) return 0; /* can't read — skip check */
+
+    long rss_mb;
+#if defined(__APPLE__) || defined(__MACH__)
+    rss_mb = (long)(ru.ru_maxrss / (1024L * 1024L));
+#else
+    rss_mb = (long)(ru.ru_maxrss / 1024L);
+#endif
+
+    if (rss_mb >= limit_mb) {
+        fprintf(stderr, "elc: memory limit exceeded (%ldMB), aborting\n", limit_mb);
+        exit(1);
+    }
+    return 0;
+}

lang/runtime/el_runtime.h

@@ -1011,6 +1011,12 @@ el_val_t  __uuid_v4(void);
 /* Args */
 el_val_t  __args_json(void);
 
+/* Compiler-support builtins — called by the El compiler's own source
+ * (compiler.el, codegen.el) and registered in codegen.el's builtin_arity. */
+el_val_t  stdout_to_file(el_val_t path);
+el_val_t  stdout_restore(void);
+el_val_t  el_mem_check(void);
+
 #ifdef __cplusplus
 }
 #endif