feat(@score/dsl): add HiHat .open() DSL method (t297)

[bwyard]

Summary

Add .open() as a chain method on HiHat, Hihat808, and Hihat909. An open hi-hat has a longer decay/release than a closed one — the cymbal rings instead of being choked. This is fundamental to every electronic genre (the open/closed hihat alternation is the backbone of house and techno grooves).

Expected API

// Closed hihat (default) — tight, choked
const hh = Hihat808(8).volume(0.3)

// Open hihat — rings out, longer decay
const hhOpen = Hihat808().open().hits(2, 10).volume(0.25)

// Or toggle per-step (advanced — planning required separately)

Where to make the change

1. packages/dsl/src/percussion.ts — add .open() method to hihat factories. Sets props.open = true in the descriptor.
2. packages/dsl/src/chain.ts — OR add as a percussion-specific extension (like makeSubSynth pattern)
3. packages/dsl/src/validators.ts — no numeric validation needed, boolean flag
4. Engine wiring — in packages/components/src/ — when props.open === true, use a longer envelope decay on the hihat

Notes

• The simplest implementation: open() sets a longer _adsr.release on the hihat descriptor — no new engine node needed
• Could also expose .decay(n) as a more general solution, but .open() is the musical name that feels right in a song file
• Must ship with tests

Good first issue — small scope, clear musical intent, no planning required.