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/audio_ring_test.cpp

@@ -0,0 +1,131 @@
+// In-process unit test for the shared audio ring (coop/audio_ring.hpp). No
+// shared memory, no hook, no audio device: it exercises the lock-free SPSC
+// push/pop, wrap-around, the format handshake, and the overrun/drop policy in a
+// single process. Exits 0 on pass, 1 on failure.
+
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <vector>
+
+#include "coop/audio_ring.hpp"
+
+using namespace coop;
+
+namespace
+{
+
+int g_failures = 0;
+
+void check(bool ok, const char* what)
+{
+	if (!ok)
+	{
+		std::printf("  FAIL: %s\n", what);
+		++g_failures;
+	}
+}
+
+// Allocate a ring of `capacity` data bytes in a plain heap buffer (placement-new
+// the header so its atomics are constructed) and return it ready to use.
+AudioRingHeader* make_ring(std::vector<std::uint8_t>& storage, std::uint32_t capacity)
+{
+	storage.assign(audio_ring_total_size(capacity), 0);
+	auto* h = new (storage.data()) AudioRingHeader();
+	audio_ring_init(*h, capacity);
+	return h;
+}
+
+} // namespace
+
+int main()
+{
+	// --- Validity + format handshake. ---
+	{
+		std::vector<std::uint8_t> storage;
+		AudioRingHeader* h = make_ring(storage, 4096);
+		check(audio_ring_valid(*h), "freshly inited ring is valid");
+		check(!audio_ring_format_ready(*h), "format not ready before set");
+		audio_ring_set_format(*h, 48000, 2, 32, 3 /*IEEE_FLOAT*/, 8);
+		check(audio_ring_format_ready(*h), "format ready after set");
+		check(h->sample_rate == 48000 && h->channels == 2 && h->bits == 32 && h->format_tag == 3 &&
+		          h->block_align == 8,
+		      "format fields round-trip");
+	}
+
+	// --- Basic push/pop integrity. ---
+	{
+		std::vector<std::uint8_t> storage;
+		AudioRingHeader* h = make_ring(storage, 4096);
+
+		std::uint8_t src[256];
+		for (int i = 0; i < 256; ++i)
+		{
+			src[i] = static_cast<std::uint8_t>(i);
+		}
+		check(audio_ring_push(*h, src, sizeof(src), 32), "push 256 bytes");
+		check(audio_ring_available(*h) == sizeof(src), "available == pushed");
+		check(h->frames_produced.load() == 32, "frames_produced tracked");
+
+		std::uint8_t dst[256] = {};
+		check(audio_ring_pop(*h, dst, sizeof(dst)) == sizeof(src), "pop returns all bytes");
+		check(std::memcmp(src, dst, sizeof(src)) == 0, "popped bytes match pushed");
+		check(audio_ring_available(*h) == 0, "ring empty after drain");
+	}
+
+	// --- Wrap-around: keep pushing/popping past capacity to force a split copy. ---
+	{
+		const std::uint32_t cap = 1024;
+		std::vector<std::uint8_t> storage;
+		AudioRingHeader* h = make_ring(storage, cap);
+
+		std::uint8_t counter = 0;
+		std::uint8_t expect = 0;
+		const std::uint32_t chunk = 300; // not a divisor of cap, so offsets drift across the seam
+		for (int iter = 0; iter < 50; ++iter)
+		{
+			std::uint8_t buf[300];
+			for (std::uint32_t i = 0; i < chunk; ++i)
+			{
+				buf[i] = counter++;
+			}
+			check(audio_ring_push(*h, buf, chunk, chunk), "wrap push fits");
+
+			std::uint8_t out[300] = {};
+			check(audio_ring_pop(*h, out, chunk) == chunk, "wrap pop full chunk");
+			bool ok = true;
+			for (std::uint32_t i = 0; i < chunk; ++i)
+			{
+				ok = ok && out[i] == expect++;
+			}
+			check(ok, "wrap data integrity across the ring seam");
+		}
+	}
+
+	// --- Overrun: a packet that doesn't fit is dropped whole, bumping overruns. ---
+	{
+		const std::uint32_t cap = 512;
+		std::vector<std::uint8_t> storage;
+		AudioRingHeader* h = make_ring(storage, cap);
+
+		std::vector<std::uint8_t> half(cap / 2, 0xAB);
+		check(audio_ring_push(*h, half.data(), cap / 2, 1), "first half fits");
+		check(audio_ring_push(*h, half.data(), cap / 2, 1), "second half fills ring");
+		check(audio_ring_available(*h) == cap, "ring full");
+
+		std::vector<std::uint8_t> more(16, 0xCD);
+		check(!audio_ring_push(*h, more.data(), 16, 1), "push into full ring rejected");
+		check(h->overruns.load() == 1, "overrun counted");
+		check(audio_ring_available(*h) == cap, "rejected push left ring untouched");
+
+		// Draining makes room again; subsequent pushes succeed.
+		std::vector<std::uint8_t> drain(cap, 0);
+		check(audio_ring_pop(*h, drain.data(), cap) == cap, "drain full ring");
+		check(audio_ring_push(*h, more.data(), 16, 1), "push succeeds after drain");
+		check(h->overruns.load() == 1, "overruns not bumped on success");
+	}
+
+	std::printf(g_failures == 0 ? "AUDIO RING TEST PASS\n" : "AUDIO RING TEST FAILED (%d)\n", g_failures);
+	return g_failures == 0 ? 0 : 1;
+}