Add mock-game capture/audio/hook stress test

mock_game_test launches coop_mock_game (DX11/DX12, frame-numbered A/V), injects
coop_hook.dll, opens the hook's shared video texture, and decodes the frame
number from the captured pixels to assert a monotonic, advancing mirror for
both backends (catches dropped/stale/out-of-order frames). Then it runs an
A/V + hook/unhook stress pass and confirms no crash + capture resumes. Adds a
read_pixel() readback to the shipping SharedTextureSource for the decode.
Robust to repeated/CI runs (kills stray games, retries inject). Trim the
roadmap (the mock-game item is done) and document the test.

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

host/src/capture/shared_texture.hpp

@@ -31,6 +31,12 @@ public:
 	// Drop the opened resources (target changed / mirror turned off).
 	void reset();
 
+	// Read the RGBA8 pixel at (x,y) of the last copied frame into out[4]. For verification
+	// (e.g. decoding the mock game's frame-number block in a capture test). Copies the
+	// private texture to a CPU-readable staging texture and maps it -- not a hot path.
+	// Returns false if no frame has been copied yet or the readback failed.
+	bool read_pixel(std::uint32_t x, std::uint32_t y, std::uint8_t out[4]);
+
 	[[nodiscard]] ID3D11ShaderResourceView* srv() const
 	{
 		return srv_.Get();