fix(tests): vision mixer FX tests — skip without a working GL context, drop fixed-sleep sampling

[srperens]

What

Both FX integration tests from #626 failed on the GitHub Actions runner (failing run). Two distinct problems, fixed in two commits:

1. The CI runner cannot create a GL context — skip there (the actual CI failure)

The runner has the GStreamer GL plugins installed, but no GL context can be created (headless, no usable software GL): a GPU pipeline builds, starts, and then silently never produces a frame. The telltale was shader_validation_test "passing" in 0.03s on the same runner — its environment probe fails fast and skips, while the FX tests only checked ElementFactory::find() (plugins present ≠ GL works).

The FX tests now use the same probe: a trivial gltestsrc ! glshader ! fakesink run with the identity fragment must reach EOS, otherwise the test skips. No CI-specific detection — the tests skip wherever GL genuinely cannot run, and keep running fully on dev boxes and GPU machines, where this coverage effectively lives.

2. Fixed sleeps made the tests flaky under load (a real local issue, fixed while diagnosing)

• vision_mixer_fx_engine_end_to_end slept a fixed 500ms after start, then took a wipe — too early and the position query behind trigger_transition fails. Now polls the mixer's position query (30s deadline) before taking.
• wipe_between_letterboxed_sources_animates sampled "mid-wipe" frames at fixed wall-clock offsets, which flaked ~1 in 3 under load when a sample landed after the wipe finished. Now watches every frame during a wipe: any frame showing both sources at once satisfies the animation assert, and observation stops once the picture settles on the incoming source. Also tightens the inverted-wipe end assert (red must be fully gone), matching the classic orientation.

A hard-switch regression still fails deterministically: saw_both never sets, the observe loop runs out its deadline, and the animation assert fires.

Also carries the workspace version bump to 0.6.2.

Testing

• Local (working GL): both tests run fully — 5+ consecutive green runs (previously flaked ~1 in 3 under load)
• CI (no GL context): both tests skip via the probe, like shader_validation_test already does

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