Recover the video mirror from a WAIT_ABANDONED keyed mutex

SharedTextureSource::update() acquired the shared-texture keyed mutex with
`AcquireSync(...) == S_OK`. But WAIT_ABANDONED -- a prior owner (e.g. a host that
crashed mid-acquire, then reconnected) died holding it -- actually GRANTS us
ownership. Treating it as failure skipped the copy AND never released, so the
next AcquireSync blocked forever and the mirror froze permanently after a crash
+ reconnect (directly relevant to the new reconnect path).

Factor the decision into keyed_mutex_acquired(HRESULT) (capture/keyed_mutex.hpp):
S_OK or WAIT_ABANDONED -> copy + release; timeout/hard errors -> skip the frame.
update() now uses it.

Test-first: keyed_mutex_test asserts WAIT_ABANDONED is treated as acquired while
the genuine "didn't get it" cases (timeout, E_FAIL, device-removed) are not. The
full cross-process abandonment is keyed-mutex OS semantics, not re-tested with a
child process here -- the predicate is the regression surface.

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

host/src/capture/keyed_mutex.hpp

@@ -0,0 +1,21 @@
+// The acquire decision for the shared video texture's keyed mutex, factored out so it can be tested
+// without a D3D device (capture/shared_texture.cpp uses it).
+#pragma once
+
+#include <windows.h> // HRESULT, S_OK, WAIT_ABANDONED, WAIT_TIMEOUT
+
+namespace coop
+{
+
+// True when an IDXGIKeyedMutex::AcquireSync result means we now HOLD the mutex and must copy + then
+// release it. S_OK is the normal case. WAIT_ABANDONED is success-with-recovery: a previous owner
+// (e.g. a host that crashed while holding it, then reconnected) terminated without releasing, so the
+// OS hands ownership to us -- treating it as a failure would skip the copy AND leak ownership, so the
+// next AcquireSync would block forever and the mirror would freeze. Timeout / hard errors mean "we
+// didn't get it, skip this frame."
+inline bool keyed_mutex_acquired(HRESULT acquire_result)
+{
+	return acquire_result == S_OK || acquire_result == static_cast<HRESULT>(WAIT_ABANDONED);
+}
+
+} // namespace coop