overlay: off-thread sljit compile worker + observability; disable sljit tier by default (emitter bug)

Off-thread sljit compile worker (overlay_compile_worker.c/.h): on a dispatch
gap-fill miss the dispatch thread snapshots the region bytes and ENQUEUES; a
background worker JITs from the snapshot (never live RAM) and publishes the shard
to the on-disk cache via an atomic temp+rename; dispatch picks it up on a later
miss via an idempotent rescan, so the candidate table stays single-threaded. This
removes the synchronous JIT frame hitch. Validated on Tomba + MMX6: compiles
off-thread (one MMX6 compile was 26ms, paid entirely off the dispatch thread with
no frame-pacing spike), shards persist + reload, queue never backs up.

Observability (ring-buffer doctrine): always-on recent-compile ring + per-compile
timing in the worker, the resolved on-disk cache dir, and a tier-enabled flag, all
surfaced via a new sljit_async TCP debug command (documented in TCP_COMMANDS.md).

DISABLE the sljit tier by default (g_sljit_tier_enabled = 0): the sljit EMITTER
mistranslates some MMX6 overlay instruction(s) -- sljit-only save-load softlocks/
loopbacks while PURE INTERP works (discriminator), and it reproduces with the
worker OFF in the synchronous path (PSX_SLJIT_SYNC=1), so it is the emitter, not
the async worker. Until the emitter bug is found + fixed: no shard READ
(scan_sljit_cache_dir skipped), no GENERATE (worker not started + sljit_live
forced 0 + dispatch gap-fill gated), every overlay miss falls to the interpreter.
gcc (DLL cache + autocompile) and overlay capture are untouched, so priority is
gcc > interp and dispatch misses are still recorded for future gcc coverage. gcc
shards are bundled in release zips, so shipped builds are unaffected.
PSX_SLJIT_ENABLE=1 re-enables the tier for emitter-fix testing.

Runtime-only, no regen.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

runtime/src/main.cpp

@@ -580,7 +580,9 @@ static std::filesystem::path default_memcard_dir(const char* argv0) {
 
 static void close_controller(void);
 
+extern "C" void overlay_compile_worker_stop(void);
 static void shutdown_runtime(void) {
+    overlay_compile_worker_stop();   /* signal + join the off-thread sljit worker */
     memcard_flush_all();
     overlay_capture_write_json();
     if (sdl_audio_device) {