Add the lynx-pretext Dancer effect as an Easter egg

[Huxpro]

Summary

Port the Dancer effect from Huxpro/lynx-pretext into BusyWeek as a hidden Easter egg.

The effect combines an animated transparent sprite with per-frame exclusion geometry. Text is laid out line by line into the space around the dancer's changing silhouette, so the content appears to flow around a moving person rather than sitting behind a video rectangle.

Easter-egg behavior

• Keep the trigger undisclosed in the normal UI.
• Suggested trigger: long-press the 好忙啊 subtitle for approximately 1.5 seconds, suppressing the normal header tap only after the long press wins.
• Present one short dancer loop over the timeline.
• During the loop, nearby Todo/intro text reflows around the dancer profile.
• A tap outside or a second trigger dismisses it immediately.
• Finishing or dismissing the effect restores the exact original timeline layout and scroll anchor.
• The Easter egg must not mutate, complete, move, or persist any Todo data.

Reference technique

The upstream example uses:

• a transparent PNG sprite sheet
• 70 sampled frames, typically played at 12 fps
• a per-frame exclusion profile divided into horizontal bands
• prepareWithSegments() and layoutNextLine()
• different available intervals on the left and right of the silhouette for each text line

Reference:

• Example: https://github.com/Huxpro/lynx-pretext/tree/main/examples/dance
• README: https://github.com/Huxpro/lynx-pretext/blob/main/examples/dance/README.md
• Implementation: https://github.com/Huxpro/lynx-pretext/blob/main/examples/dance/src/index.tsx

BusyWeek adaptation

• Render the dancer in a non-interactive overlay inside the timeline viewport.
• Generate a temporary line/exclusion layout from current visible content without changing stored data.
• Prefer reflowing a decorative intro/copy layer first; reflow actual Todo rows only if identity, hit targets, and accessibility remain correct.
• Keep checkbox/delete hit targets outside the animated exclusion layout or disable them for the short duration.
• Pause frame advancement when the app is backgrounded or the overlay is not visible.
• Cache prepared text and exclusion data; do not re-prepare text every frame.

Asset and performance constraints

• Confirm the license and redistribution status of any upstream dancer sprite before copying it.
• If the existing character assets are unsuitable for redistribution, reproduce the technique with an original BusyWeek mascot/dancer asset.
• Keep the shipped asset size intentional and documented.
• Target sprite updates at the source cadence (for example 12 fps); do not force 60 text-layout recomputations per second.
• Instrument frame time on representative iOS and Web targets before enabling the Easter egg in production.

Acceptance criteria

• A hidden, deterministic gesture enters and exits the Easter egg without conflicting with the completed-items header toggle.
• The dancer animates from a sprite sheet on Web and Lynx native.
• Visible text flows around the per-frame silhouette instead of a fixed rectangle.
• No Todo state, ordering, completion, or persistence changes.
• Dismissal restores the previous layout and scroll position without a flash.
• The overlay does not permanently block Todo gestures or accessibility.
• Backgrounding/unmounting stops timers and animation work.
• Reduced-motion mode uses a still silhouette or a simple non-reflowing appearance.
• Asset licensing/attribution and bundle-size impact are documented.
• Tests cover trigger arbitration, teardown, frame selection, exclusion intervals, and state preservation.