feat(@score/dsl): theory.ts — transpose(), relativeMinor(), dominantOf(), parallelMajor(), modeOf() (t240)

[bwyard]

Summary

Add five music theory utility functions to packages/dsl/src/theory.ts. These are used in song files to build progressions, transpose ideas, and explore harmonic relationships without memorising interval tables.

Expected API

import { scale, chord, transpose, relativeMinor, dominantOf } from '@score/dsl'

// Transpose a note or array of notes by semitones
transpose('A2', 7)           // → 'E3'
transpose(['C3','E3','G3'], 5) // → ['F3','A3','C4']

// Relative minor of a major key
relativeMinor('C')   // → 'A'
relativeMinor('G')   // → 'E'

// Dominant (V chord root) of a key
dominantOf('C')      // → 'G'
dominantOf('Am')     // → 'E'

// Parallel major of a minor key (same root, major mode)
parallelMajor('Am')  // → 'A'

// Mode of a scale
modeOf('C', 2)       // → 'D Dorian'
modeOf('C', 5)       // → 'G Mixolydian'

Where to make the change

• packages/dsl/src/theory.ts — add the five functions
• packages/dsl/src/index.ts — export them
• packages/gui/src/renderer/types/score-dsl.d.ts — ambient declarations for Monaco

Notes

• All functions are pure — same input, same output, no side effects
• transpose should handle note strings like 'A2', 'C#3', 'Bb4'
• Must ship with unit tests covering edge cases (sharps, flats, octave boundaries)
• TSDoc required on all five exports

Good first issue — pure functions, well-defined music theory rules, no engine wiring needed.