timeline editor: quantize grid, drag fixes, hotkeys

Fixes:
- Sequence dragging with scroll offset
- Double-click collapse/expand (DOM recreation issue)
- Collapsed sequence dragging (removed stopPropagation)

Features:
- Quantize dropdown (Off, 1/32→1 beat) replaces snap-to-beat checkbox
- Works in both beat and second display modes
- Hotkeys: 0=Off, 1=1beat, 2=1/2, 3=1/4, 4=1/8, 5=1/16, 6=1/32
- Separate "Show Beats" toggle for display vs snap behavior

Technical:
- Track dragMoved state to avoid unnecessary DOM recreation
- Preserve dblclick detection by deferring renderTimeline()
- Quantization applies to sequences and effects uniformly

handoff(Claude): timeline editor quantize + drag fixes complete

1 files changed, 9 insertions, 5 deletions

diff --git a/tools/timeline_editor/README.md b/tools/timeline_editor/README.md
index 4fcb2f4..cc13a41 100644
--- a/tools/timeline_editor/README.md
+++ b/tools/timeline_editor/README.md
@@ -14,7 +14,7 @@ Interactive web-based editor for `timeline.seq` files.
 - ⚙️ Stack-order based priority system
 - 🔍 Zoom (10%-200%) with mouse wheel + Ctrl/Cmd
 - 🎵 Audio waveform visualization (aligned to beats)
-- 🎼 Snap-to-beat mode (enabled by default)
+- 🎼 Quantize grid (Off, 1/32, 1/16, 1/8, 1/4, 1/2, 1 beat)
 - 🎛️ BPM slider (60-200 BPM)
 - 🔄 Re-order sequences by time
 - 🗑️ Delete sequences/effects
@@ -37,10 +37,11 @@ Interactive web-based editor for `timeline.seq` files.
    - Watch sequences auto-expand/collapse during playback
    - Red playback indicators on both timeline and waveform show current position
 5. **Edit:**
-   - Drag sequences/effects to reposition
-   - Double-click sequence header to collapse/expand
+   - Drag sequences/effects to reposition (works when collapsed or expanded)
+   - Double-click anywhere on sequence to collapse/expand
    - Click item to edit properties in side panel
    - Drag effect handles to resize
+   - **Quantize:** Use dropdown or hotkeys (0-6) to snap to grid
 6. **Zoom:** Ctrl/Cmd + mouse wheel (zooms at cursor position)
 7. **Save:** Click "💾 Save timeline.seq"
 
@@ -102,7 +103,9 @@ open "tools/timeline_editor/index.html?seq=../../workspaces/main/timeline.seq"
 ## Keyboard Shortcuts
 
 - **Spacebar**: Play/pause audio playback
+- **0-6**: Quantize grid (0=Off, 1=1beat, 2=1/2, 3=1/4, 4=1/8, 5=1/16, 6=1/32)
 - **Double-click timeline**: Seek to position (continues playing if active)
+- **Double-click sequence**: Collapse/expand
 - **Ctrl/Cmd + Wheel**: Zoom in/out at cursor position
 
 ## Technical Notes
@@ -113,10 +116,11 @@ open "tools/timeline_editor/index.html?seq=../../workspaces/main/timeline.seq"
 - BPM used for seconds conversion (tooltips, audio waveform alignment)
 - Priority determines render order (higher = on top)
 - Collapsed sequences show 35px title bar, expanded show full effect stack
-- Time markers show beats by default (4-beat/bar increments)
+- **Show Beats** toggle: Switch time markers between beats and seconds
+- Time markers show 4-beat/bar increments (beats) or 1s increments (seconds)
 - **Waveform and time markers are sticky** at top during scroll/zoom
 - Vertical grid lines aid alignment
-- Snap-to-beat enabled by default for musical alignment
+- **Quantize grid**: Independent snap control (works in both beat and second display modes)
 - **Auto-expand/collapse**: Active sequence expands during playback, previous collapses
 - **Auto-scroll**: Timeline follows playback indicator (keeps it in middle third of viewport)
 - **Dual playback indicators**: Red bars on both timeline and waveform (synchronized)