docs: formalize the reactive contract (trigger keys, frozen parameters, cancel-and-restart)

[l2hyunwoo]

Summary

#84 made key-driven recompilation work. What is missing is a written contract for what callers may rely on. Three rules, each verified against the runtime source:

1. Every value read inside content is frozen at compile time. The internal composition is created, composed once, and disposed (Jindong.kt:120-127); there is no recomposition path. Keys are the only recompile trigger.
2. Keys are playback triggers. Put state in keys exactly when its change should fire playback. A slider value passed as a key fires a vibration on every drag step; state that only shapes the pattern belongs inside content. The sample app hit this distinction in review (feat: rebuild sample app as an 8-screen haptic verification harness #86).
3. Cancel-and-restart is best effort. A key change cancels the in-flight playback before starting the new one, with ordering guaranteed by the manager's state lock (HapticManager.kt:126-149). Neither Vibrator.cancel() nor CHHapticPatternPlayer.stop reports completion, so a few milliseconds of physical overlap are possible.

Scope

• KDoc on Jindong plus a documentation page covering the three rules.
• Document the value-embedding idiom Jindong(pattern) { Clip(pattern) }. A Jindong(pattern, vararg keys) overload was prototyped and dropped: Jindong(p) { ... } resolves to the existing vararg overload with p as a key, which compiles and silently plays the lambda instead of the pattern.
• Regression tests: a parameter change without a key must not re-fire; a key change must.

Related: #56 covers suppressing playback on initial composition, which belongs to the same contract.

Depends on #90.