feat(core): add deterministic keyframe ease runtime

[miguel-heygen]

What

Add the deterministic easing runtime used by Studio-authored keyframes, including custom curves, hold, spring, and wiggle easing.

This is Family A, PR 1 of 4. It is rooted on main; the next PR is codex/studio-timeline-a-parser-intent-v2.

Replacement review history: #2561. Golden reference: #2387.

Why

Studio must interpret and render every authored ease identically in preview and capture. Keeping the easing parser and runtime registration together gives the stack one deterministic owner for those semantics.

How

The runtime registers the supported custom-ease forms once, exposes the shared motion-ease model through Core, and adds focused coverage for parsing and runtime initialization. Current main already contains the merged runtime bridge from #2558; this PR does not recreate it.

Test plan

• Unit tests added/updated — 79 focused Core tests passed at this PR boundary
• Manual testing performed — covered by the certified integrated Studio QA matrix before stack materialization
• Documentation updated (not applicable; internal runtime and Studio behavior)

[miguel-heygen]

• fix(parsers): preserve duration-authored keyframe timing #2675
• feat(studio): add keyframe ease editor #2674
• fix(parsers): preserve authored keyframe intent #2673
• feat(core): add deterministic keyframe ease runtime #2672 👈 (View in Graphite)
• main

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