fix(elc): add el_mem_check() memory guard — abort before OS OOM-kill

Add el_mem_check() to el_runtime.c: reads ELC_MAX_MEM_MB (default 512),
checks RSS via getrusage (macOS bytes / Linux KB normalised to MB), prints
a clear diagnostic to stderr and exits(1) if exceeded.

Wire it into two places:
- compiler.el: upfront check at --emit-header entry point
- codegen.el: per-function check in the streaming loop after each
  el_arena_pop, so runaway growth is caught at the earliest function
  boundary rather than after the machine is already dying.

lang/el-compiler/runtime/el_runtime.c

@@ -42,6 +42,7 @@
 #include <dirent.h>
 #include <errno.h>
 #include <pthread.h>
+#include <sys/resource.h>  /* getrusage — memory guard */
 #ifdef HAVE_CURL
 #include <curl/curl.h>
 #endif
@@ -5674,6 +5675,50 @@ el_val_t getpid_now(void) {
     return (el_val_t)getpid();
 }
 
+/* el_mem_check — self-terminating memory guard for long-running compiler runs.
+ *
+ * Call this periodically (e.g. after each function compiled) to detect runaway
+ * memory growth before the OS OOM-killer fires. Reads the limit from the env
+ * var ELC_MAX_MEM_MB (default 512 MB). If resident set size exceeds the limit,
+ * prints a diagnostic to stderr and exits with code 1 so the caller (elb or a
+ * CI script) can handle the failure gracefully instead of having the whole
+ * machine go down.
+ *
+ * Platform notes:
+ *   macOS  — ru_maxrss is in bytes.
+ *   Linux  — ru_maxrss is in kilobytes.
+ * We normalise to MB before comparing.
+ *
+ * Returns 0 always (the only non-return path is the exit() branch).
+ */
+el_val_t el_mem_check(void) {
+    /* Read limit from env; default 512 MB. */
+    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__)
+    /* macOS: ru_maxrss is bytes */
+    rss_mb = (long)(ru.ru_maxrss / (1024L * 1024L));
+#else
+    /* Linux: ru_maxrss is kilobytes */
+    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;
+}
+
 /* ── args() — command-line argument access ──────────────────────────────────
  * Compiled El programs call args() to get a list of CLI arguments.
  * Call el_runtime_init_args(argc, argv) at the start of C main() to populate.

lang/el-compiler/runtime/el_runtime.h

@@ -531,6 +531,12 @@ el_val_t  parse_int(el_val_t s, el_val_t default_val);
 el_val_t  exit_program(el_val_t code);
 el_val_t  getpid_now(void);
 
+/* Self-terminating memory guard. Reads ELC_MAX_MEM_MB (default 512) and
+ * exits with code 1 if resident memory exceeds the limit. Call periodically
+ * during long compilation loops (e.g. after each function is compiled).
+ * Returns 0 when memory is within bounds.                                   */
+el_val_t  el_mem_check(void);
+
 /* ── CGI identity ─────────────────────────────────────────────────────────────
  * Called at the start of main() in CGI programs (those with a `cgi {}` block).
  * Records the program's DHARMA identity before any other code executes.      */

lang/el-compiler/src/codegen.el

@@ -3730,6 +3730,7 @@ fn codegen_streaming(tokens: [Any], sigs: [Map<String, Any>], source: String) ->
                             cg_fn(stmt)
                             el_release(stmt)
                             el_arena_pop(fn_arena_mark)
+                            el_mem_check()
                         }
                     } else {
                         if is_top_level_decl(stmt) {

lang/el-compiler/src/compiler.el

@@ -571,6 +571,7 @@ fn main() -> Void {
     // BinOp chains (e.g. checkout.el) — parse() builds O(whole-program AST)
     // while scan_fn_sigs_el keeps peak memory at O(tokens).
     if do_emit_header {
+        el_mem_check()
         let raw_source: String = fs_read(src_path)
         let hdr_tokens: [Any] = lex(raw_source)
         let hdr_sigs: [Map<String, Any>] = scan_fn_sigs_el(hdr_tokens)