feat(scenes): storyboard passthrough for scrubber previews

[karngyan]

What

Adds storyboard support to the scenes provider so kino's scrubber shows thumbnail previews on hover for scene sequences, exactly as it already does for Mux sources.

• ScenesProviderOptions (and therefore ScenesPlayer) gain storyboard?: { vttUrl }. When set, the provider reports capabilities.hasStoryboard: true and state.storyboard = { vttUrl }; when absent, behavior is unchanged (false / null).
• SceneManifest gains an optional storyboard?: string field recording where the thumbnail VTT lives. It is read by embedding players, not by the host.
• README scenes section documents the option; changeset included (minor).

Why

The scrubber already consumes state.storyboard.vttUrl plus capabilities.hasStoryboard and parses Mux-style thumbnail VTTs (#xywh fragments into a sprite image). The scenes provider only needed to surface the same shape. The karnstack ship pipeline generates the sprite grid and VTT per sequence and uploads them alongside the bundle; the caller passes the (possibly token-bearing) VTT URL in, and kino never touches the auth layer.

Testing

• New provider tests: storyboard option maps to capability + state; absent option keeps hasStoryboard: false and storyboard: null.
• Full gate: pnpm typecheck && pnpm lint && pnpm test && pnpm format:check && pnpm build (157 tests, 24 files, all green).

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	kino	6fdf319	Commit Preview URL Branch Preview URL	Jul 20 2026, 07:21 PM