Reduce filesize of spring and inertia generators

[mattgperry]

Summary

Filesize pass over the spring and inertia generators: -374 bytes minified / -107 bytes gzipped (4,773 → 4,399 min, 2,185 → 2,078 gzip, measured by bundling spring + inertia + pregenerateKeyframes with esbuild).

• spring: The underdamped position/velocity math existed in three places (resolveSpring, resolveVelocity, and an inlined copy in the next() hot path). It now exists once, in a sample(t) closure memoized by t that computes both values while sharing the exp/sin/cos terms — same one-trig-evaluation-per-frame behavior, single-path next(). Also hoists the repeated dampingRatio * undampedAngularFreq product and replaces Math.pow(x, 2) with multiplication in findSpring. Output is bit-identical (spring tests pass unchanged).
• inertia: applyFriction calculates calcDelta once per frame instead of twice. The boundary spring now receives the exact analytical derivative of the friction curve (-calcDelta(t) / timeConstant per ms) instead of a 5ms finite-difference sample, removing calcLatest and the getGeneratorVelocity import and resolving the stale units TODO on that line. isOutOfBounds relies on comparisons against undefined always being false.
• generators/utils: calcGeneratorDuration now accepts optional timeStep, maxDuration and keyframes parameters so pregenerateKeyframes reuses the same sampling loop instead of duplicating it.

Behaviour notes

• The analytical inertia boundary velocity shifts bounce trajectories by at most 0.033 units on the 200-unit test animation (~0.016%), only after the boundary crossing; durations unchanged. The pinned keyframe expectations in inertia.test.ts were regenerated and verified programmatically (values before the boundary are bit-identical, min/max cases deviate symmetrically).
• A generator that never rests now caps pregeneration at exactly 10,000ms (one extra keyframe) instead of 9,990ms.

Test plan

• motion-dom Jest suite (67 suites, 509 tests)
• framer-motion client Jest suite (97 suites, 815 tests)
• ESLint clean on changed files

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