Audio render-hook M2: render hook + in-process self-test (concept proven)

Implements the WASAPI render-hook (hook/src/audio_hook.{hpp,cpp}) and an
in-process self-test that proves COM vtable discovery and GetBuffer/ReleaseBuffer
interception with no game and no second Steam account.

- audio_hook.cpp: anchors on IMMDevice::Activate (idx 3) off our own default
  endpoint (shared vtable), then hooks IAudioClient::Initialize (3) /
  GetService (14) and IAudioRenderClient::GetBuffer (3) / ReleaseBuffer (4) off
  live game pointers. Copies primary-stream frames into the audio ring and
  releases with AUDCLNT_BUFFERFLAGS_SILENT (+ memset belt-and-suspenders), only
  while the host-owned capture_enabled flag is set. Stream counting runs always;
  on a ring overrun it keeps playing locally rather than going silent.
- ipc_client.hpp: publish_audio_stream / note_audio_frames /
  set_audio_streams_seen write the render-stream debug fields into HookStatus.
- tests/audio_hook_test.cpp: installs the hooks, renders a tone through WASAPI
  in-process, and asserts exactly one stream, frames pushed to the ring, the
  ring carries the non-silent tone, and the primary was silenced. PASS:
  streams_seen=1, frames_captured=32640.
- plan doc: correct GetService vtable index 13 -> 14 (SetEventHandle is 13).

coop_hook DLL wiring + host consumer/fallback come next (M3).

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

tests/CMakeLists.txt

@@ -34,3 +34,24 @@ target_link_libraries(audio_loopback_test PRIVATE mmdevapi ole32)
 
 add_dependencies(audio_loopback_test coop_tone)
 add_test(NAME audio_loopback_test COMMAND audio_loopback_test)
+
+# In-process self-test for the WASAPI render-hook. Reuses the shipping
+# audio_hook.cpp and drives a real WASAPI render path in the same process, so it
+# exercises COM vtable discovery + GetBuffer/ReleaseBuffer interception with no
+# game and no second Steam account (the audio analogue of hook_selftest).
+add_executable(audio_hook_test
+	audio_hook_test.cpp
+	${CMAKE_SOURCE_DIR}/hook/src/audio_hook.cpp)
+
+target_include_directories(audio_hook_test PRIVATE ${CMAKE_SOURCE_DIR}/hook/src)
+
+# IAudioClient3 / process-audio APIs want the Windows 10 20H1 (NTDDI_WIN10_CO) headers.
+target_compile_definitions(audio_hook_test PRIVATE NTDDI_VERSION=0x0A00000B)
+
+target_link_libraries(audio_hook_test PRIVATE
+	coop_common
+	safetyhook::safetyhook
+	ole32
+	mmdevapi)
+
+add_test(NAME audio_hook_test COMMAND audio_hook_test)