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>

common/include/coop/protocol.hpp

@@ -11,7 +11,7 @@ namespace coop
 
 // Bump whenever the layout of SharedBlock or CoopPadState changes. The hook
 // refuses to attach to a host with a mismatched version.
-inline constexpr std::uint32_t kProtocolVersion = 3;
+inline constexpr std::uint32_t kProtocolVersion = 4;
 
 // 'COOP' little-endian, used to sanity-check the mapping before trusting it.
 inline constexpr std::uint32_t kProtocolMagic = 0x504F4F43u;
@@ -42,6 +42,24 @@ struct CoopPadState
 
 static_assert(sizeof(CoopPadState) == 20, "CoopPadState layout must stay stable across both modules");
 
+// Maximum render streams the diagnostics track. The hook captures only the
+// first ("primary"); the rest are surfaced so a multi-stream game is visible.
+inline constexpr std::uint32_t kMaxAudioStreams = 4;
+
+// One render stream the hook observed, for the Audio panel's debug view. Plain
+// POD (no atomics): diagnostics tolerate benign cross-process races like the
+// other HookStatus counters. frames_rendered is cumulative; the host derives
+// "live vs idle" from successive deltas.
+struct AudioStreamInfo
+{
+	std::uint32_t is_primary;   // 1 = the stream the hook captures/silences
+	std::uint32_t sample_rate;
+	std::uint16_t channels;
+	std::uint16_t bits;
+	std::uint32_t format_tag;   // WAVE_FORMAT_* of this stream
+	std::uint64_t frames_rendered;
+};
+
 // Indices into HookStatus::focus_query_calls.
 enum FocusApi : std::uint32_t
 {
@@ -73,6 +91,12 @@ struct HookStatus
 	std::uint32_t raw_input_gamepad;      // ... for a joystick/gamepad usage page
 	std::uint32_t raw_input_gamepad_sink; // ... and that usage has RIDEV_INPUTSINK (bg delivery)
 	std::uint32_t dinput_loaded;          // dinput8.dll is present in the process
+
+	// Audio render-hook diagnostics. Stream counting runs whenever the DLL is
+	// injected, independent of whether audio mirroring is enabled, so a
+	// multi-stream game is visible before/without turning the mirror on.
+	std::uint32_t audio_streams_seen;                 // distinct render clients ever created
+	AudioStreamInfo audio_streams[kMaxAudioStreams];  // per-slot detail, [0] is primary
 };
 
 // Top-level shared block. The host is the sole writer of pad state; the hook is