feat(@score/dsl): add .autopan(rate) chain method

[bwyard]

Summary

Add .autopan(rate) as a chain method. Auto-pan oscillates the stereo position left-to-right at rate Hz. Creates movement and width in a mix — classic in house, trance, and ambient.

Expected API

// Slow pan sweep on a pad
Pad('A3').attack(0.4).autopan(0.5).volume(0.5)

// Fast rhythmic pan on a hihat
Hihat808(8).autopan(2).volume(0.3)

Where to make the change

1. packages/dsl/src/chain.ts — add readonly autopan: (rate: number) => T to ChainMethods<T>, implement in createPart()
2. packages/dsl/src/validators.ts — rate > 0
3. packages/gui/src/renderer/types/score-dsl.d.ts — Monaco ambient declaration
4. Engine wiring — LFO → StereoPannerNode

Notes

• Depth defaults to full sweep (-1 to 1) for simplicity. A depth param could be added later.
• LFO infrastructure is in @score/modulation
• Must ship with tests and TSDoc

Good first issue — same pattern as .tremolo() (#126), fully self-contained.