feat(scheduler): add reusable Jelly spring

[Xuepoo]

What and why

Bakudan needs a real renderer-agnostic squash/stretch contract before its Jelly control can ship. This adds deterministic spring state to preallocated pool slots without introducing UI, Canvas, DOM, or per-frame allocations.

Changes

• add preallocated/reset jellyScaleX, jellyScaleY, and jellyVelocity slot state
• add bounded Scheduler.exciteJelly() and showcaseJelly integration
• excite new comments and pointer repulsion while preserving existing preset motion
• document the public API and add a minor changeset

Public API and behavior compatibility

New required PoolSlot fields and Scheduler controls are additive. Jelly is disabled by default; disabled schedulers reset active slots to exact neutral state. Existing preset position, rotation, and opacity semantics remain unchanged.

Verification

• bun run format:check
• bun run lint
• bun test — 47 pass, 0 fail, 504 assertions
• bun run build
• npm pack --dry-run

Checklist

• Added a Changeset for a public package/API/behavior change.
• Updated README.md.
• Added deterministic pool, scheduler, and preset tests.
• Kept the package renderer-agnostic and independent from VectoJS, Canvas, DOM, and browser layers.
• Wrote documentation and non-obvious code comments in English.