repeat, yoyo, steps easing, and a declarative camera shake

[LeadcodeDev]

Part of #326. Batch 2 — depends on #336 for TimePoint.

Reproducing the reel's camera shake took 155 sampled camera keyframes, generated by a Python loop evaluating a damped sine. GSAP writes the same motion as {x: 7, duration: .035, repeat: 11, yoyo: true}. AnimationTiming.repeat is a bool with no count, there is no yoyo, and EasingType has no steps, so a blinking caret and an oscillating offset both have to be spelled out keyframe by keyframe.

Owned files

• crates/rustmotion-core/src/schema/animation.rs (EasingType::Steps, repeat widening)
• crates/rustmotion-core/src/schema/video.rs (AnimationTiming)
• crates/rustmotion-core/src/schema/shake.rs (new), crates/rustmotion-core/src/engine/shake.rs (new)
• crates/rustmotion-core/src/engine/animator.rs (modifier resolution — coordinate with Animate scalars that other attributes derive from #329, which owns variable tracks in the same file)

Do NOT write: schema/scenario.rs (#336 registers Scene.shake), cli/commands/geometry.rs (#334).

Deliverables

1. repeat: n — an integer count, with true still meaning infinite. This widens an existing bool, so the deserializer accepts both forms and nothing breaks.

2. yoyo: bool and repeat_delay, on any animation rather than on a fixed set of presets.

3. EasingType::Steps(n) — the blinking caret and anything else that must jump rather than interpolate.

4. Scene.shake — { impacts: [{ at, amplitude }], decay, frequency, rotation }, where at is a TimePoint so impacts can be written on the beat grid. Additive over camera, so a pan and a shake coexist rather than fighting over the same keyframe track.

5. PostEffect::Flash { at, color, intensity, duration }, which costs almost nothing once impacts exist and is what the reference reel pairs with every impact.

Acceptance

• A six-impact shake written as six {at, amplitude} pairs replaces 155 hand-generated keyframes and renders the same motion
• repeat: true on an existing scenario behaves exactly as it does today
• A caret on steps(1) holds each state rather than fading between them

[LeadcodeDev]

Delivered on main at 187cf7c (squash of #340). Verified present on main, not taken on the PR's word.