add exec() and exec_bg() builtins to El runtime

- exec(cmd) -> String: runs shell command, captures stdout, 30s timeout
- exec_bg(cmd) -> String: forks command in background, returns PID string
- add both to codegen arity table (builtin_arity)
- rebuild elc with updated arity table (self-hosting, identity-verified)
- update release snapshot at releases/v1.0.0-20260501/

el-compiler/runtime/el_runtime.h

@@ -742,6 +742,8 @@ el_val_t  map_set(el_val_t map, el_val_t key, el_val_t value); /* el_map_set */
 /* ── Subprocess execution ────────────────────────────────────────────────── */
 el_val_t exec_command(el_val_t cmd);   /* run shell command, return exit code */
 el_val_t exec_capture(el_val_t cmd);   /* run shell command, capture stdout   */
+el_val_t exec(el_val_t cmd);           /* exec(cmd) → stdout String (30s timeout) */
+el_val_t exec_bg(el_val_t cmd);        /* exec_bg(cmd) → PID String (non-blocking) */
 
 el_val_t emit_log(el_val_t level, el_val_t msg, el_val_t fields_json);
 el_val_t emit_metric(el_val_t name, el_val_t value, el_val_t tags_json);