[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/kernel/xboxkrnl/xboxkrnl_video.cc

@@ -9,6 +9,7 @@
 
 #include "xenia/kernel/xboxkrnl/xboxkrnl_video.h"
 
+#include "xenia/base/guest_liveness.h"
 #include "xenia/base/logging.h"
 #include "xenia/emulator.h"
 #include "xenia/gpu/graphics_system.h"
@@ -472,6 +473,10 @@ void VdSwap_entry(
     lpdword_t frontbuffer_ptr,  // ptr to frontbuffer address
     lpdword_t texture_format_ptr, lpdword_t color_space_ptr, lpdword_t width,
     lpdword_t height) {
+  // Guest-progress heartbeat for the hang watchdog: the game asking to present
+  // a frame is our "still alive" signal (diagnostics only).
+  xe::liveness::guest_swaps.fetch_add(1, std::memory_order_relaxed);
+
   // All of these parameters are REQUIRED.
   assert(buffer_ptr);
   assert(fetch_ptr);