Audio render-hook M1: shared audio ring + protocol diag fields

First milestone of the injection render-hook audio path (see
docs/audio-render-hook-plan.md) that fixes the local audio echo without a
virtual device.

- common/include/coop/audio_ring.hpp: new lock-free SPSC shared-memory ring
  for PCM, separate from the input/status SharedBlock. Free-running 64-bit
  positions (release/acquire), format handshake, host-owned capture_enabled
  gate, drop-whole-packet overrun policy.
- common/include/coop/protocol.hpp: add AudioStreamInfo + audio_streams_seen /
  audio_streams[] to the always-present HookStatus for the render-stream-count
  debug view; bump kProtocolVersion 3->4 (new members appended).
- tests/audio_ring_test.cpp: in-process unit test (push/pop integrity,
  wrap-around, format handshake, overrun/drop). No hook or audio device.
- docs/audio-render-hook-plan.md: the green-lit design this implements.

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

tests/CMakeLists.txt

@@ -13,6 +13,12 @@ target_link_libraries(hook_selftest PRIVATE
 
 add_test(NAME hook_selftest COMMAND hook_selftest)
 
+# Unit test for the shared audio ring (lock-free SPSC push/pop, wrap-around,
+# format handshake, overrun policy). Header-only, no hook or audio device.
+add_executable(audio_ring_test audio_ring_test.cpp)
+target_link_libraries(audio_ring_test PRIVATE coop_common)
+add_test(NAME audio_ring_test COMMAND audio_ring_test)
+
 # Integration test for WASAPI process-loopback capture. Reuses the shipping
 # capture code and captures from coop_tone (a known sine-wave render process).
 add_executable(audio_loopback_test