Capture every audio stream into its own ring and mix them on the host

Games with several concurrent WASAPI render streams (e.g. Spider-Man: Miles
Morales) only had their first ("primary") stream mirrored; the rest kept playing
locally and never reached the guest. Now the render-hook captures + silences EVERY
tracked stream into its own ring (coop_audio_<pid>[_<index>]), each published with
that stream's own detected format (Initialize when caught, else GetMixFormat -- the
per-stream format detection, now actually used per ring rather than only for the
primary). The host creates a ring per stream and mixes the same-format streams with
a soft clip (host/src/audio/audio_mix.hpp); streams whose format differs from the
primary are still silenced (no echo) but skipped from the mix (would need
resampling).

The single-stream case is byte-for-byte unchanged: when only one stream is active
the host passes it through without the mixer, so the common path has no overhead or
fidelity change.

Verified: new audio_mix_test covers the decode/sum/soft-clip/encode math (float32 +
int16); audio_hook_test (x64 + x86) still passes, guarding the primary
capture+silence path against regression; full build x64 + x86 clean; ctest x64
11/11, x86 3/3. Multi-stream mixing against a real multi-stream game needs a live
session to fully confirm.

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

host/src/audio/audio_loopback.hpp

@@ -18,6 +18,7 @@
 #include <windows.h>
 
 #include "coop/audio_ring.hpp"
+#include "coop/protocol.hpp" // kMaxAudioStreams
 #include "coop/shared_memory.hpp"
 
 namespace coop
@@ -97,8 +98,9 @@ public:
 private:
 	void thread_main(DWORD pid);
 	// Returns true if it owned the session to a clean stop; false if setup failed
-	// and the caller should fall back to the loopback path.
-	bool run_hooked(AudioRingHeader* ring);
+	// and the caller should fall back to the loopback path. `rings[0]` is the primary
+	// stream; additional non-null rings are mixed in.
+	bool run_hooked(AudioRingHeader* const* rings);
 	void run_loopback(DWORD pid);
 	bool wait_for_format(AudioRingHeader* ring, DWORD timeout_ms);
 	bool stop_requested() const;
@@ -108,7 +110,7 @@ private:
 	HANDLE stop_event_ = nullptr;
 	DWORD pid_ = 0;
 
-	SharedMemory audio_ring_shm_; // host-created shared ring (named coop_audio_<pid>)
+	SharedMemory audio_ring_shm_[kMaxAudioStreams]; // per-stream rings (coop_audio_<pid>[_<i>])
 
 	std::atomic<bool> running_{false};
 	std::atomic<Source> source_{Source::None};