Split useMidiPlayer.ts into focused modules + grow frontend tests

[TwDover]

Split useMidiPlayer.ts into focused modules + grow frontend tests

Break the 1228-line useMidiPlayer.ts composable (1228 -> 610) into focused,
mostly-testable modules — the offline WAV render, shared constants, synth
voices, voice routing, and the scheduler trigger callbacks — and cover the
fragile extracted logic with tests (96 -> 123).

New modules (pure moves — no behaviour change):

• composables/playerConstants.ts — style-classification Sets +
  PLAYER_PARTS/PlayerPart/CHANNEL_PART, so live playback and the offline
  render read one source (no import cycle). useMidiPlayer re-exports
  PLAYER_PARTS/PlayerPart so external importers are unaffected.
• composables/useOfflineRender.ts — the parallel per-part WAV export
  (offlineRenderRaw, sumPartBuffers, partHasNotes, limitMix, renderOfflineFast,
  isRendering). The pause-on-export stays a thin wrapper in useMidiPlayer,
  which owns the playback refs.
• soundfonts/synthVoices.ts — the 7 live-playback synth voice factories (next
  to synthDrums.ts); they take the player's disposables array so cleanup still
  disposes their nodes.
• composables/voiceRouting.ts — the pure melodic voice-selection decision
  (resolveMelodicVoiceKind) + CC10 pan (panFromCC10); getMelodicInstrument now
  switches on the returned kind.
• composables/scheduler.ts — the Tone.Part per-note trigger callbacks
  (drumTriggerCallback/voiceTriggerCallback): mute gating, the kick sidechain
  pump, and trigger args, injected into toggle so they test without a Tone
  context.

New tests: wavEncoder (RIFF header / interleave / clamp / mono), voiceRouting
(voice-selection precedence + pan), scheduler (mute gating, pump, trigger args),
and partHasNotes channel->part mapping.

Verified: vue-tsc + eslint + production build + 123 vitest green. Live playback

• WAV export smoke-tested in the desktop app.