feat(skills): baked-sim rule — offline physics stepped at build time, replayed by frame index

[jbernard077]

Adds one rule to skills/hyperframes-animation: organic physics (a rope, flag, cloth, debris, a
flock) is simulated once at build time and replayed by frame index, so the renderer never runs
an integrator. Validated in a deterministic reference build (a verlet pennant in a 24 s reel) through
the local gate.

What's added

• rules/baked-sim.md — the bake → ship → replay contract:
  • a build script steps the sim at dt = 1/fps with a seeded PRNG, fixed iteration counts and
    per-frame forces computed from the frame number, after a ~60-frame pre-roll so frame 0 is
    already settled;
  • the output ships as a synchronous script include (window.__NAME = {...}) — never
    fetch(), which races the first-frame capture;
  • playback is one ease: "none" driver spanning exactly LAST / fps seconds so the proxy equals the
    frame index at every sampled frame; its onUpdate does frames[round(f)] — an index lookup and
    nothing else — with the frame-0 state applied at setup.
  • Recipe (verlet rope bake + SVG path replay), variations (transform arrays, canvas draw, two-rate
    bake, re-bake for art direction), values, constraints.
• A rules-index.md row and a boundary sentence in particle-burst.md (interacting bodies are
  baked; ballistic particles stay closed-form), plus the hook-regenerated skills-manifest.json.

Validation

Evidence	Result
hyperframes check on the reel	0 errors (one pre-existing structural lint warning — four scenes on one track)
Double-render frame hashes (PNG sequence, static-frame dedup off)	720/720 bit-identical
Seek-shuffle (shuffled-order direct seeks vs ascending order, including t = 17.0 s — the pennant scene's first frame — and 3 more inside that scene)	9/9 bit-identical
Bake determinism	seeded mulberry32(42), dt = 1/30, 15 constraint passes, 60-frame pre-roll; re-running the bake reproduces the data file byte for byte, and the rule's own recipe run as a script reproduces it frame for frame (both logged)
Runtime audit	no Math.random, Date.now, or requestAnimationFrame in the composition; the replay is a single index lookup

Fit against current HEAD

Anchors re-verified against main @ 250afc3. The rule sits beside particle-burst (closed-form,
no bake) and points at ../adapters/three.md for the GPU lane; it introduces no new contract —
it is the existing determinism rules applied to stateful physics.

Preview media

(the pennant scene of the reference reel: verlet rope baked offline, replayed by index, end card on
a critically damped spring)

pr-b-baked-pennant.mp4

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