Keep Metal presentation active in mixed IOSurface layer trees

docs/TESTING.md

@@ -90,6 +90,7 @@ Useful probes (build tokenized Python helpers from `build/runtime/tools/`):
 - `m327-cgl-resolver-check`: first calls through dynamically looked-up GL functions retain floating-point arguments; expected `first_sampler_linear=1 first_color_arguments=1 errors=0,0`. Build with `build-m58f327-opengl.py` and run with `run-riot-native-check.py m327-cgl-resolver-check`.
 - `m327-cgl-direct-check`: with `RCLOL_GPU_DIRECT_OTHER_WINDOW_TEST=1` and an owned 337x339 native target, interleave 338x339 fallback surfaces; expected `active_presenter_preserved=1 fallback_frames=357`. This needs the native presentation fixture; it is not a `run-riot-native-check.py` choice.
 - `riot-metal-presentation-check`: Vulkan clear reaches the GL texture, including late window/context attachment. Set `RIOT_PRESENTATION_NESTED=1` to place the Metal layer beneath two ordinary layers and verify that its drawables are still presented and recycled past the three-drawable pool limit.
+- `riot-ca-render-policy-check`: headless check of the actual compositor render policy. A nested Metal layer must still be prepared when a full-window IOSurface is present; surface-only trees should retain the local-render shortcut. Built by `build-riot-metal-presentation.py`; expected `PASS: mixed-tree Metal preparation survives IOSurface shortcut`. Creates no window or GPU work and does not publish its test surface.
 - `riot-metal-concurrency-check`: submission ordering, resource lifetime and concurrent recording.
 
 `RIOT_PRESENTATION_STRESS=1` makes the presentation probe run 120 frames. With the game and clients closed, force async encoding for this standalone probe using the guest flag `/tmp/metal-async-encode`, then remove it. The tested old path leaked 120 synchronization FDs over 120 frames; the fixed path leaked zero. Inspect the probe's host `/proc/PID/fd` for `anon_inode:syncobj_file` if reproducing that test.