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>

tests/CMakeLists.txt

@@ -25,6 +25,11 @@ add_executable(mkb_ring_test mkb_ring_test.cpp)
 target_link_libraries(mkb_ring_test PRIVATE coop_common)
 add_test(NAME mkb_ring_test COMMAND mkb_ring_test)
 
+# Unit test for the audio mixer math (decode/sum/soft-clip/encode). Header-only.
+add_executable(audio_mix_test audio_mix_test.cpp)
+target_include_directories(audio_mix_test PRIVATE ${CMAKE_SOURCE_DIR}/host/src)
+add_test(NAME audio_mix_test COMMAND audio_mix_test)
+
 # Unit test for the host->game mouse coordinate mapping (letterbox inverse +
 # decorated-window client offset). Header-only, no device.
 add_executable(mkb_map_test mkb_map_test.cpp)
@@ -148,6 +153,7 @@ coop_output_subdir(tests
 	audio_ring_test
 	mkb_ring_test
 	mkb_map_test
+	audio_mix_test
 	audio_loopback_test
 	audio_hook_test
 	srgb_format_test