Add semantic motion engine

[raiseCatError]

Implements #172 (engine and shimmer migration; see Deferred).

Stacked on #221 (feature/165-surfaces), on #220, #219, #218, #217 and #203. Merge order: #203, #217, #218, #219, #220, #221, then this PR.

What changed

• src/motion/motion.ts: a pure engine. sampleMotion(profile, elapsedMs, cell) returns a 0..1 intensity. Shapes are sine (breathing/traveling shimmer), comet (slow rise, fast fall), tail (fast rise, long tail) and pulse (settle). Spread is uniform or traveling. Easing comes from Chroma's curves.
• Profiles per semantic state: waiting, processing, streaming, transition, completion, failure. One-shot profiles settle at zero and do not repeat.
• Reduced motion (NMSH_REDUCED_MOTION=1 or deterministic presentation) holds every profile still.
• The shimmer now samples the processing and streaming profiles. Its output is unchanged, and transcript activity rows now also honor reduced motion.
• The engine has no timers, no width changes and no PTY access.

Deferred

• No shared scheduler/clock was added. The one existing activity timer is already bounded (100 ms), cleared on exit, and its renders are suppressed during passthrough. A scheduler should arrive with a second animated consumer.
• waiting, transition, completion and failure have no consumer yet. The issue asks not to animate merely because the engine exists.

Verification

tests/motion.test.ts and the existing tests/shimmer.test.ts; full suite 656/656, typecheck, build and git diff --check clean.