[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/apu/audio_watchdog.h

@@ -0,0 +1,104 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2026 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_APU_AUDIO_WATCHDOG_H_
+#define XENIA_APU_AUDIO_WATCHDOG_H_
+
+#include <atomic>
+#include <cstdint>
+
+// Read-only diagnostics for "audio stops mid-mission and never recovers".
+//
+// The audio pipeline has four stages, each of which can die independently:
+//
+//   guest XAudio callback  ->  SubmitFrame  ->  ring buffer  ->  ALSA write
+//        (guest_pumps)       (frames_submitted)              (alsa_writes)
+//                     XMA decoding feeds the guest (xma_works)
+//
+// Each stage bumps a counter here. The watchdog thread in audio_system.cc
+// samples them once a second and, when audio dies, reports which stage stopped
+// advancing -- turning "no sound" into a named culprit.
+//
+// Note the APU worker runs the guest's audio callback IN-LINE
+// (AudioSystem::WorkerThreadMain -> processor_->Execute). If that guest call
+// ever blocks, pumping stops forever and audio can never recover, so we also
+// track how long we have been inside guest code.
+//
+// Everything here is counters only: cvar-gated (--audio_watchdog), no
+// behaviour change.
+
+namespace xe {
+namespace apu {
+namespace watchdog {
+
+// Pumps of the guest XAudio callback (AudioSystem worker).
+inline std::atomic<uint64_t> guest_pumps{0};
+// steady_clock microseconds at which the worker entered guest code; 0 = not
+// currently inside the guest callback.
+inline std::atomic<uint64_t> in_guest_callback_since_us{0};
+// Frames the guest handed to the driver (AudioSystem::SubmitFrame).
+inline std::atomic<uint64_t> frames_submitted{0};
+// XMA decoder Work() calls that actually decoded something.
+inline std::atomic<uint64_t> xma_works{0};
+// ALSA: periods of real guest audio written, silence keepalive periods
+// written, and underruns recovered.
+inline std::atomic<uint64_t> alsa_writes{0};
+inline std::atomic<uint64_t> alsa_silence{0};
+inline std::atomic<uint64_t> alsa_xruns{0};
+// Last observed snd_pcm_state_t (-1 = unknown / driver not ALSA).
+inline std::atomic<int> alsa_state{-1};
+
+// Why is the APU worker not pumping? Three very different causes look the same
+// from outside the process, so distinguish them here:
+//   worker_loops == 0 && clients_in_use == 0 -> no client (guest unregistered)
+//   worker_loops == 0 && clients_in_use  > 0 -> parked despite a client (pacing)
+//   worker_loops  > 0 && pump_skips     > 0 -> looping, but no free output slot
+inline std::atomic<uint64_t> worker_loops{0};
+inline std::atomic<uint64_t> pump_skips{0};
+inline std::atomic<int> clients_in_use{-1};
+inline std::atomic<uint64_t> register_calls{0};
+inline std::atomic<uint64_t> unregister_calls{0};
+
+// Exactly where the APU worker thread is sitting. A parked worker and a worker
+// blocked on the kernel global lock are indistinguishable from outside the
+// process (both are futex waits with zero context switches), so record it.
+enum class WorkerPhase : int {
+  kStart = 0,
+  kAcquiringGlobalLock,  // blocked here => a kernel lock is held elsewhere
+  kParkedNoClient,       // blocked here => guest has no audio client
+  kPacingSleep,          // normal: waiting for the next 5.33ms deadline
+  kInGuestCallback,      // normal: running the game's mixer
+  kSubmitting,
+};
+inline std::atomic<WorkerPhase> worker_phase{WorkerPhase::kStart};
+
+inline const char* WorkerPhaseName(WorkerPhase p) {
+  switch (p) {
+    case WorkerPhase::kStart:
+      return "start";
+    case WorkerPhase::kAcquiringGlobalLock:
+      return "ACQUIRING-GLOBAL-LOCK";
+    case WorkerPhase::kParkedNoClient:
+      return "PARKED-NO-CLIENT";
+    case WorkerPhase::kPacingSleep:
+      return "pacing";
+    case WorkerPhase::kInGuestCallback:
+      return "in-guest-callback";
+    case WorkerPhase::kSubmitting:
+      return "submitting";
+    default:
+      return "?";
+  }
+}
+
+}  // namespace watchdog
+}  // namespace apu
+}  // namespace xe
+
+#endif  // XENIA_APU_AUDIO_WATCHDOG_H_