refactor: timeline editor now uses beat-based internal storage

Timeline editor now stores all times internally as beats (not seconds),
aligning with the project's beat-based timing system. Added BPM slider
for tempo control. Serializes to beats (default format) and displays
beats primarily with seconds in tooltips.

Changes:
- parseTime() returns beats (converts 's' suffix to beats)
- serializeSeqFile() outputs beats (bare numbers)
- Timeline markers show beats (4-beat/bar increments)
- BPM slider (60-200) for tempo editing
- Snap-to-beat rounds to nearest beat
- Audio waveform aligned to beats
- showBeats enabled by default

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

1 files changed, 11 insertions, 6 deletions

diff --git a/tools/timeline_editor/README.md b/tools/timeline_editor/README.md
index 4861a88..dd1f38b 100644
--- a/tools/timeline_editor/README.md
+++ b/tools/timeline_editor/README.md
@@ -5,16 +5,17 @@ Interactive web-based editor for `timeline.seq` files.
 ## Features
 
 - 📂 Load/save `timeline.seq` files
-- 📊 Visual Gantt-style timeline with sticky time markers
+- 📊 Visual Gantt-style timeline with sticky time markers (beat-based)
 - 🎯 Drag & drop sequences and effects
 - 🎯 Resize effects with handles
 - 📦 Collapsible sequences (double-click to collapse)
 - 📏 Vertical grid lines synchronized with time ticks
-- ⏱️ Edit timing and properties
+- ⏱️ Edit timing and properties (in beats)
 - ⚙️ Stack-order based priority system
-- 🔍 Zoom (10%-500%) with mouse wheel + Ctrl/Cmd
-- 🎵 Audio waveform visualization
-- 🎼 Snap-to-beat mode
+- 🔍 Zoom (10%-200%) with mouse wheel + Ctrl/Cmd
+- 🎵 Audio waveform visualization (aligned to beats)
+- 🎼 Snap-to-beat mode (enabled by default)
+- 🎛️ BPM slider (60-200 BPM)
 - 🔄 Re-order sequences by time
 - 🗑️ Delete sequences/effects
 
@@ -73,8 +74,12 @@ SEQUENCE 2.5s 0 "Explicit seconds"   # Rare: start at 2.5 physical seconds
 ## Technical Notes
 
 - Pure HTML/CSS/JavaScript (no dependencies, works offline)
-- Sequences have absolute times, effects are relative to parent sequence
+- **Internal representation uses beats** (not seconds)
+- Sequences have absolute times (beats), effects are relative to parent sequence
+- 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)
 - Time markers sticky at top when scrolling
 - Vertical grid lines aid alignment
+- Snap-to-beat enabled by default for musical alignment