Honour asset.speed in preview playback

[dazzatronus]

Closes #145.

asset.speed round-tripped through updateClip/getEdit and rendered correctly, but canvas preview always played at 1x. This makes preview playback, the toolbar, and timeline resizing speed-aware so preview matches the rendered output.

Playback parity. Renders map clip time to source time as speed × (trim + t) — trim is scaled by speed too, and audio is resampled (pitch shifts). Preview now uses the same mapping: playbackRate/Howler rate() set from asset.speed, all seeks and drift correction target the speed-adjusted source time, preservesPitch disabled to match the resampled render. speed: 0 previews as a frozen frame with no audio, as rendered. Applies to video and audio clips.

Toolbar speed control. New control in the media toolbar (video/audio): log-scaled slider centred on 1x, presets 0.25x–4x, editable value input, range 0.1–10. Changing speed rescales clip length and trim by oldSpeed/newSpeed so the clip keeps the same source window at the new pace; auto/end length intents are preserved. One undo entry per change.

Speed-aware resize cap. Timeline resize stops where source media runs out (duration/speed − trim, via new Player.getMaxLength()). Growth-only: clips already longer than the source window keep their length (renders freeze the last frame past source end, so over-long clips stay valid).

Verify: load src/templates/speed.json, play — segment pace and pitch should match a render of the same JSON. Change speed on a clip via the toolbar and confirm the clip rescales; drag a video clip's right edge and confirm it stops at the source end. npx jest (1886 tests) covers the mapping, max-length maths, and toolbar rescale.

[github-actions]

🎉 This PR is included in version 2.15.0 🎉

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