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/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)