[WIP] Audio/threading fixes + crash investigation; NEW ORACLE: crash is ours not the game

Snapshot for handoff. Contains the mission-audio + threading fixes and the
crash-investigation instrumentation (all diagnostic cvars default-OFF).

Fixes (behavioral):
- threading_posix.cc: reap-once guard on PosixCondition<Thread>::post_execution
  (double pthread_join at mission teardown -> fault loop -> audio death + freeze).
- xma_decoder.cc: work_event_->Set() in Pause() so the idle XMA worker observes
  paused_ and signals pause_fence_ (Pause() deadlock -> permanent audio death).
- audio_system / xma_context_master / xboxkrnl_audio / apu_flags / alsa: mission
  audio keepalive + guest_audio_flags + watchdogs.

Instrumentation (additive, default-off): xboxkrnl_debug cache-throw diag +
guest-catch dispatcher, xex_module PE/PDATA/EH scans, kernel_state mem_watch
(NOTE: mem_watch DEFAULTS TRUE -- an always-on host poll thread; prime crash suspect).

NEW ORACLE (see HANDOFF-crash-oracle-2026-07-16.md): stock 6e5b8324f built with
our toolchain + zero custom code = NO crash, NO sound-stop, plays the Ready Room.
=> the Ready-Room out_of_range crash is introduced by THESE changes, not the game
and not the (LTO-broken) build chain. Bisection plan + suspect ranking in the note.

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

src/xenia/emulator.h

@@ -10,11 +10,13 @@
 #ifndef XENIA_EMULATOR_H_
 #define XENIA_EMULATOR_H_
 
+#include <atomic>
 #include <cstddef>
 #include <cstdint>
 #include <functional>
 #include <optional>
 #include <string>
+#include <thread>
 #include <vector>
 
 #include "xenia/apu/audio_media_player.h"
@@ -309,6 +311,12 @@ class Emulator {
   xe::Delegate<> on_exit;
 
  private:
+  // Hang watchdog (--hang_watchdog_secs): when the guest stops presenting
+  // frames, dump every guest thread's registers + guest stack. Diagnostics.
+  void DumpGuestHang(uint64_t stalled_s);
+  std::atomic<bool> hang_watchdog_running_{false};
+  std::thread hang_watchdog_thread_;
+
   enum : uint64_t { EmulatorFlagDisclaimerAcknowledged = 1ULL << 0 };
   static uint64_t GetPersistentEmulatorFlags();
   static void SetPersistentEmulatorFlags(uint64_t new_flags);