feat(audio): add Formant Envelope Follower

[ford442]

Adds a native Web Audio envelope follower in FormantShifter.ts (using WaveShaperNode for rectification and BiquadFilterNode for smoothing) to dynamically modulate formant shift amounts based on vocal amplitude. This adds natural "blooming" effects to the synthesized vocals.

Also exposes global controls in SamplerPanel and per-step overrides in NoteSelector, and cleans up duplicate setters and duplicate variables inside FormantShifter.ts and SingingVoice.ts.

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PR created automatically by Jules for task 3748415928042466324 started by @ford442

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Reviewing files that changed from the base of the PR and between d9d45b7 and 0c4fb49.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (8)

• agent_plan.md
• package.json
• pnpm-workspace.yaml
• src/components/SamplerPanel.tsx
• src/engines/SingingVoice.ts
• src/engines/rubberband/FormantShifter.ts
• src/hooks/useAudioEngine.ts
• src/types.ts

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[ford442]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in f8dd603.