feat(@score/dsl): add .tremolo(rate, depth) chain method

[bwyard]

Summary

Add .tremolo(rate, depth) as a chain method on all instruments. Tremolo is amplitude modulation — the volume oscillates at rate Hz with depth controlling the intensity (0–1). Essential for vintage organ, guitar, and lo-fi sounds.

Expected API

// Tremolo on a pad — slow, subtle wobble
Pad('C3').attack(0.3).tremolo(4, 0.3).volume(0.5)

// Faster tremolo on a plucked string
Pluck('E4').tremolo(8, 0.6).volume(0.7)

Where to make the change

1. packages/dsl/src/chain.ts — add readonly tremolo: (rate: number, depth: number) => T to ChainMethods<T> interface, implement in createPart()
2. packages/dsl/src/validators.ts — add validation: rate > 0, depth 0–1
3. packages/gui/src/renderer/types/score-dsl.d.ts — add to ambient declarations for Monaco IntelliSense
4. Engine wiring in packages/components/ — connect _tremolo descriptor prop to a TremoloNode (LFO → GainNode)

Notes

• Follow the pattern of existing fx chain methods: .chorus(), .phaser() in chain.ts
• The LFO infrastructure is already in @score/modulation — use it
• Must ship with tests and TSDoc
• Rate is in Hz (not BPM-synced — that's a separate tremoloSync concern)

Good first issue — self-contained chain method addition, clear scope, no planning required.