- Import CommandHistory and all 4 command classes from new modules
- hitTest() with 8px EDGE for resize-left/resize-right/move zones
- _hasOverlap() check for time range collision on same track
- snapToBeat() with 0.1s threshold and calibrationOffset
- _setupInteractions() with mousedown/mousemove/keydown listeners
- _startMove(): live drag preview, snap on mouseup, overlap rejection
- _startResize(): left/right edge handles, 0.5s min duration, snap
- handleDrop(): place block from palette with crypto.randomUUID()
- Keyboard: Delete, Ctrl+Z (undo), Ctrl+Shift+Z / Ctrl+Y (redo)
- app.js: dragstart on palette, dragover/dragleave/drop on canvas
- Ghost block rendered during dragover for visual feedback
- Resize handle indicators drawn on selected block
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