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>

README.md

@@ -95,16 +95,6 @@ and covers anything the hooked path doesn't (Vulkan, D3D9 — see Roadmap).
 
 ## Roadmap
 
-### Planned (next up)
-
-- **Mock game + stress-test suite.** A bare-bones test game that renders an animated
-  (non-static) pattern — so dropped/duplicated/torn frames are obvious — with a
-  selectable graphics backend (DX11/DX12 now; structured to add OpenGL/Vulkan) and a
-  configurable audio output (rate/channels/bit-depth/format, like `coop_tone`). Then a
-  comprehensive suite that drives it through the real capture + audio + hook paths:
-  verify both capture backends, every audio variant, hook/unhook cycles, and that
-  nothing races or crashes under stress.
-
 ### Future work
 
 - **Vulkan video hook.** Vulkan games present via `vkQueuePresentKHR`; hooking
@@ -217,6 +207,17 @@ ctest --test-dir build -C Debug --output-on-failure
   process-loopback capture (the fallback backend) receives non-silent audio by PID for
   each — confirming loopback is format-agnostic (it captures post-mix at the device
   endpoint format). Skips cleanly if the machine has no audio endpoint.
+- **`mock_game_test`** — comprehensive capture/audio/hook stress test against
+  **`coop_mock_game`** (an animated, frame-numbered A/V test game under
+  [`tools/mock_game`](tools/mock_game) with selectable **DX11 / DX12** backends and a
+  configurable WASAPI tone). It launches the game, injects `coop_hook.dll`, opens the
+  hook's shared video texture, and **decodes the frame number out of the captured pixels**
+  to assert the mirror sees a *monotonic, advancing* sequence for both backends (the bar
+  for no dropped / stale / out-of-order frames — what the DX12 rotating-backbuffer bug
+  broke). It then injects with audio + video, checks both stream, cycles the audio
+  subsystem off/on (hook/unhook stress), and confirms the game never crashes and capture
+  resumes. This suite drove out five real audio races (see Lessons learned). Skips cleanly
+  without a D3D11 device.
 
 ### Debugging the hooks against a real game