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.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.      */