fix(player): avoid black flash at playback startup

[stackia]

Fix the brief black flash after the first picture when picture enhancement or deinterlacing is enabled. This fixes a startup regression introduced in #755.

The first video-frame callback can arrive while readyState is still HAVE_METADATA, so primeCanvas() skips drawing. The callback now renders that decoded frame when startup left the texture ring empty, instead of exposing an empty canvas until the next frame. Successfully primed frames still avoid a duplicate upload and filter pass.

Validation: five focused regression tests cover enhancement, deinterlacing, both enabled, already-primed startup, and raw-video fallback. The three affected cases fail before the fix. All 48 frontend tests, TypeScript, changed-file Biome checks, and the production frontend build pass. Six Chrome/WebGL playback cases using generated fixtures, including progressive and 1080i H.264/MP2 MSE playback, show no black frames after the first picture and no GL errors. The low-frame-rate fixture reproduces the black flash before the fix. E2E lint and both collection entry points also pass (626 tests collected; daemon tests were not run for this frontend-only change).

Fixes #760.

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Note

Low Risk
Narrow change to first-frame scheduling in the video render loop, with regression tests; no auth, data, or API surface changes.

Overview
Fixes a startup black flash when picture enhancement or deinterlacing is on: the first requestVideoFrameCallback can fire before readyState reaches HAVE_CURRENT_DATA, so primeCanvas() skips and the canvas stays empty until the next frame.

scheduleFrame now calls processFrame when the texture ring is still empty (textures.length === 0), not only when the loop was already running (wasRunning). Frames that were already primed still skip a second upload/filter pass.

Adds renderer.test.ts with Vitest coverage: first-frame present for enhancement/deinterlace combos, no double upload when startup is already primed, and no GL path when both processing features are off (mocked presenters/GPU).

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