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.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