[players] Render video viewers through a canvas pipeline

[frankrousseau]

Problems

• Paused seeks could paint a different frame than the one announced — the historical source of off-by-one annotation placements.
• The visible <video> elements blocked any per-pixel display work (LUTs) and forced the playlist player to juggle element visibility.
• QA surfaced old player bugs: entity fps ignored on two surfaces (12fps duplicates), laser strokes never fading, stroke caps lost, tile thumbnails drifting, Alt+arrow navigation double-handled.

Solutions

• Decode in hidden <video> elements, paint through a visible canvas on every requestVideoFrameCallback tick, in VideoViewer and MultiVideoViewer; frameRenderer.js is the seam for a future WebGL/LUT backend. Overlays, wheel and comparison anchor on the canvas.
• Fix the surfaced bugs: per-entity fps everywhere, laser fade, round caps, measured tile-sprite geometry, single-handled Alt+arrow, no playlist reload on untouched add panel close.
• Restyle the playlist progress strip and the trim handles; generate timeline stripes at exact frame size; add wheel zoom with a mini-map to the timeline for long clips.
• Gates: 936 unit tests, prod build, full manual QA.