test: add Tier 1 pure helper tests (fx, envelope, midi, waveform, mixer, piano)

[808StaN]

Summary

• Add unit tests for pure helper modules across audio, utils, components and playlist:
  • src/audio/domain/fxParams.js — EQ, reverb, maximizer params + soft clip curve
  • src/audio/domain/envelope.js — ADSHR envelope scheduling on mock AudioParam
  • src/audio/core/getNormalizeGain.js — peak scan, caching, multi-channel
  • src/utils/midiPattern.js — drag payload build/parse/round-trip
  • src/utils/midiImport.js — MIDI file name detection, ArrayBuffer parsing, DataTransfer helpers
  • src/utils/patternNotes.js — step/piano note merging and sorting
  • src/utils/sampleUrl.js — pack URL encoding and normalization
  • src/components/mixer/mixerUiUtils.js — effect names, insert labels, percent formatting
  • src/components/piano-roll/pianoRollUtils.js — pitch/velocity math, quantize, scale steps
  • src/components/playlist/playlistWaveformUtils.js — waveform envelope, SVG path, clip window math

Additional changes

• Added DataTransfer global mock to src/test/setup.js for drag-and-drop tests in jsdom.

Verification

npm run test:run
# Test Files  27 passed (27)
# Tests  327 passed (327)