Recover a guessed audio stream's rate by correlating hook vs loopback (step a)

When the host attaches to an already-running game it never saw the stream's
Initialize, so the render-hook assumes the device mix format and measures only
the sample rate from the render cadence -- which a jittery game can make wrong
(intermittent pitch shift). But during the measurement window the game is still
audible, so we have the same audio twice: the hook (pre-mix, unknown format) and
a process-loopback (post-mix, the known device format). Cross-correlating them
pins the true rate from ground truth.

- common/include/coop/audio_correlate.hpp: the pure correlator. Resample the hook
  by each candidate standard rate up to the device rate and score how well it
  aligns with the loopback across the window (drift-detecting). audio_correlation_test
  recovers every rate (score ~1.0 vs ~0.01 for wrong ones), incl. 44100-vs-48000,
  and rejects unrelated signals.
- Hook measurement tap: a host-set verify_capture ring flag makes the hook push a
  still-being-measured (guessed) stream's raw pre-mix bytes WITHOUT silencing, so
  the host can co-capture both signals (a silenced game's loopback is silent).
  Inert by default -- the shipping no-echo path is untouched.
- host/src/audio/audio_format_verifier: co-captures hook + loopback and correlates,
  feeding a correction into the existing override channel. Wired into AudioMirror's
  measurement window (hidden in the gap loopback already covers, so exact streams
  pay nothing). audio_verify_test drives it end-to-end against coop_mock_game.

Rate vs layout are coupled (correlating the waveform needs the right channel
de-interleaving), so this step assumes the hook layout matches the device (the
common stereo-on-stereo case); recovering a different channel count / bit depth
is step b.

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

tools/audio_validate/CMakeLists.txt

@@ -8,6 +8,7 @@
 add_executable(coop_audio_validate
 	main.cpp
 	${CMAKE_SOURCE_DIR}/host/src/audio/audio_loopback.cpp
+	${CMAKE_SOURCE_DIR}/host/src/audio/audio_format_verifier.cpp
 	${CMAKE_SOURCE_DIR}/host/src/audio/process_loopback_capture.cpp
 	${CMAKE_SOURCE_DIR}/host/src/audio/audio_overrides.cpp)