feat: timeline editor audio playback with auto-expand/collapse

- Audio playback controls (play/pause, spacebar shortcut)
- Red playback indicator with auto-scroll (middle third viewport)
- Auto-expand active sequence during playback, collapse previous
- Click waveform to seek
- Sticky header: waveform + timeline ticks stay at top
- Sequences confined to separate scrollable container below header
- Document known bugs: zoom sync, positioning, reflow issues

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 19 insertions, 5 deletions

diff --git a/tools/timeline_editor/README.md b/tools/timeline_editor/README.md
index dd1f38b..6e368cf 100644
--- a/tools/timeline_editor/README.md
+++ b/tools/timeline_editor/README.md
@@ -18,19 +18,26 @@ Interactive web-based editor for `timeline.seq` files.
 - 🎛️ BPM slider (60-200 BPM)
 - 🔄 Re-order sequences by time
 - 🗑️ Delete sequences/effects
+- ▶️ **Audio playback with auto-expand/collapse** (NEW)
+- 🎚️ **Sticky audio track and timeline ticks** (NEW)
 
 ## Usage
 
 1. **Open:** `open tools/timeline_editor/index.html` or double-click in browser
 2. **Load timeline:** Click "📂 Load timeline.seq" → select `workspaces/main/timeline.seq`
-3. **Edit:**
+3. **Load audio:** Click "🎵 Load Audio (WAV)" → select audio file
+4. **Playback:**
+   - Click "▶ Play" or press **Spacebar** to play/pause
+   - Click waveform to seek
+   - Watch sequences auto-expand/collapse during playback
+   - Red playback indicator shows current position
+5. **Edit:**
    - Drag sequences/effects to reposition
    - Double-click sequence header to collapse/expand
    - Click item to edit properties in side panel
    - Drag effect handles to resize
-4. **Zoom:** Ctrl/Cmd + mouse wheel (zooms at cursor position)
-5. **Audio:** Load WAV file for waveform visualization
-6. **Save:** Click "💾 Save timeline.seq"
+6. **Zoom:** Ctrl/Cmd + mouse wheel (zooms at cursor position)
+7. **Save:** Click "💾 Save timeline.seq"
 
 ## File Format
 
@@ -71,6 +78,11 @@ SEQUENCE 2.5s 0 "Explicit seconds"   # Rare: start at 2.5 physical seconds
   EFFECT + Fade 0 4                  # Still uses beats for duration
 ```
 
+## Keyboard Shortcuts
+
+- **Spacebar**: Play/pause audio playback
+- **Ctrl/Cmd + Wheel**: Zoom in/out at cursor position
+
 ## Technical Notes
 
 - Pure HTML/CSS/JavaScript (no dependencies, works offline)
@@ -80,6 +92,8 @@ SEQUENCE 2.5s 0 "Explicit seconds"   # Rare: start at 2.5 physical seconds
 - 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)
-- Time markers sticky at top when scrolling
+- **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
+- **Auto-expand/collapse**: Active sequence expands during playback, previous collapses
+- **Auto-scroll**: Timeline follows playback indicator (keeps it in middle third of viewport)