Audio verify: distinguish a failed process-loopback from no-correlation

verify_stream_format ignored ProcessLoopbackCapture::start()'s bool. A failed
loopback activation then produced an empty ground-truth signal, so the result was
ok=false -- indistinguishable from "captured fine but the two paths didn't
correlate" -- after burning the whole measurement window capturing only the hook
side for nothing.

Now it checks start(): on failure it restores the ring tap, emits a clear
OutputDebugString diagnostic, and returns immediately (ok=false) instead of
wasting the window. The caller still falls back to the measured guess, but the
cause is now visible.

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

README.md

@@ -114,8 +114,6 @@ From an in-depth review pass. Each item is fixed test-first (a failing test, the
 as its own commit; "verify" items are confirmed real before any change, and dropped if not.
 
 Robustness (verify, then fix if real):
-- **Swallowed audio loopback `start()` failures** — surface a process-loopback activation failure
-  distinctly from "no correlation" in the verifier + loopback.
 - **Permissive injector bitness gate** — `IsWow64Process2` failure is treated as 64-bit; fall back to
   `IsWow64Process` instead of mis-injecting.
 - **Ignored HRESULT/BOOL returns** — `CreateShaderResourceView`, `GetClientRect`/`ClientToScreen`,

host/src/audio/audio_format_verifier.cpp

@@ -156,12 +156,20 @@ FormatVerification verify_stream_format(DWORD pid, AudioRingHeader* ring, unsign
 	std::vector<BYTE> loop_bytes;
 	ProcessLoopbackCapture loop;
 	const std::uint32_t loop_block = dev_wfx->nBlockAlign;
-	loop.start(pid, dev_wfx, [&](const BYTE* data, std::uint32_t frames, bool silent) {
+	if (!loop.start(pid, dev_wfx, [&](const BYTE* data, std::uint32_t frames, bool silent) {
 		    if (!silent && data != nullptr)
 		    {
 			    loop_bytes.insert(loop_bytes.end(), data, data + static_cast<std::size_t>(frames) * loop_block);
 		    }
-	});
+	    }))
+	{
+		// Distinguish "couldn't activate process loopback" from "captured fine but didn't correlate":
+		// without the ground-truth post-mix path there's nothing to correlate against, so bail now
+		// (don't burn the window capturing only the hook side) and leave the diagnostic visible.
+		OutputDebugStringA("coop: verify_stream_format -- process-loopback activation failed; cannot verify format\n");
+		ring->verify_capture.store(0, std::memory_order_release);
+		return result;
+	}
 
 	// Pull the hook's pre-mix bytes out of the ring across the window.
 	std::vector<BYTE> hook_bytes;