feat(sampler): add Custom Sample Slicing UI with transient visualization and dragging

[ford442]

This PR implements the "Custom Sample Slicing UI" from the agent_plan.md Active Backlog.

Features

• Interactive Slicing UI: WaveformDisplay now allows users to manually define and manipulate sample slice markers when in stretch mode for the Sampler.
• Double-Click Interactions: Users can double-click anywhere on the waveform canvas without an existing slice to initialize one covering the entire file, double-click inside an existing slice to split it in two at that point, or double-click near an existing transient marker to merge adjacent slices.
• Draggable Boundaries: Hovering over a marker changes the cursor to col-resize and clicking and dragging adjusts the boundary between two slices natively.
• Audio Engine Integration: The useAudioEngine hook now exports a setAlignment function, passed down to SamplerPanel and mapped to onAlignmentChange, bridging the React state update of these boundaries with the underlying PhonemeAligner representations so they function identically to TTS syllables.
• Roadmap Update: The agent_plan.md has been successfully updated, marking this feature complete and moving the logical next step ("Auto-Slice by Transients") to the Innovation Lab.

Verification

• Fully verified via pnpm test, pnpm lint, and pnpm build.
• Merged with main to ensure clean resolution.

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PR created automatically by Jules for task 14867844128597551224 started by @ford442

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