fix(vision mixer): a restarted border frame is due on the mixer's next output

[srperens]

Problem

vision_mixer_underlay_upload_test::static_underlays_are_not_reuploaded_gpu fails now and then on the macOS runner: the configured border is missing from the cut frame: (0, 0, 255). Seen on #1029's CI after its rebase, and in 1 of 5 runs from a diagnostic branch earlier today.

In production, on a host where the vision mixer renders below real time, a border configured while the mixer is behind shows up late, after the take that should reveal it.

A border colour change, or a border starting to hold, restarts its underlay source, and the restart stamped the new frame with the clock's running time. A mixer that renders slower than real time falls further behind the clock every second (the GL mixer on the runner manages about 20 fps of a 30 fps flow). The frame then lies in the mixer's future and is not shown until the mixer gets there.

Change

gst::underlay::frame_time stamps the restarted frame at the compositor's output position (query_position on the aggregator the source feeds) plus 50 ms. The margin keeps it after the held frame, which a held pad would otherwise drop as from the past. The stamp is never later than the clock's running time, which is the old stamp and is where a mixer that keeps up already is. A held pad (repeat-after-eos) keeps a frame stamped in its past, so the frame is shown on the next output frame.

Evidence

• New guard borders_keep_up_with_a_mixer_behind_the_clock_gpu: the same flow with the PGM mixer held 60 ms per output frame (test-only BUFFER probe), so it falls behind the clock as on the runner. With the old stamp it fails with the CI signature, the configured border is missing from the cut frame: (0, 0, 255), and did so 2 of 2 times before the fix. With the fix it passes, and the colour change shows after 2 PGM frames (122 ms).
• The existing GPU and CPU variants pass unchanged.

Tests

Ran (macOS, GStreamer 1.28.6, GL): STROM_REQUIRE_GL=1 cargo test --test vision_mixer_underlay_upload_test (3 passed), cargo test --lib underlay (2), vision_mixer_cpu_test, vision_mixer_overlay_upload_test, vision_mixer_source_resize_test, pipeline_lifecycle_test: all passed. cargo clippy --all-targets -D warnings clean.
Not run: Linux and Windows (CI). This PR carries ci:macos.

For the reviewer

• The new test adds about 8 s to vision_mixer_underlay_upload_test.

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