docs: Add Task #57 - Interactive Timeline Editor (low priority)

Added new low-priority task for creating a web-based interactive editor
for demo.seq files. This would evolve the existing Gantt chart HTML output
into a full interactive editor.

Features:
- Load/save demo.seq files
- Visual Gantt-style timeline
- Drag & drop sequences and effects
- Edit timing and properties
- Overlap detection
- Zoom/pan navigation

Technical scope:
- Pure HTML/JavaScript tool
- No backend or code integration
- No preview rendering
- Standalone editor for timeline files

Priority: Very Low (quality of life improvement)
Effort: 1-2 weeks for basic functionality

This addresses the need for faster iteration on demo timing without
recompiling, and better visualization of sequence overlaps.

1 files changed, 46 insertions, 0 deletions

diff --git a/TODO.md b/TODO.md
index 0170cec..aecc06a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -363,4 +363,50 @@ src/tests/
 
 ---
 
+## Low Priority: Developer Tools & Editors
+
+### Task #57: Interactive Timeline Editor (Gantt Chart Evolution)
+**Goal:** Create a web-based interactive editor for `demo.seq` files based on the existing Gantt chart HTML output.
+
+**Features:**
+- **Load/Save**: Parse and serialize `demo.seq` files
+- **Visual Timeline**: Interactive Gantt-style view of sequences and effects
+- **Drag & Drop**: Move sequences and effects along the timeline
+- **Edit Timing**: Adjust start/end times, priorities
+- **Overlap Detection**: Visual feedback for sequence/effect conflicts
+- **Zoom/Pan**: Navigate long timelines easily
+- **Effect Properties**: Edit constructor arguments inline
+
+**Technical Scope:**
+- Pure HTML/JavaScript tool (no backend needed)
+- Loads `demo.seq` text file via file input
+- Interactive visualization using D3.js or similar
+- Saves modified `demo.seq` back to disk
+- **Does NOT**: Interact with C++ code, preview rendering, compile timeline
+
+**File Structure:**
+```
+tools/timeline_editor/
+├── index.html           # Main editor UI
+├── editor.js            # Core editing logic
+├── parser.js            # demo.seq parser
+├── serializer.js        # demo.seq writer
+├── timeline.js          # Gantt chart renderer
+└── README.md            # Usage instructions
+```
+
+**Use Cases:**
+- Quickly adjust demo timing without recompiling
+- Visualize sequence overlaps and priorities
+- Experiment with different timing arrangements
+- Export modified timeline for integration
+
+**Priority:** Very Low (quality of life improvement, not critical)
+
+**Effort:** 1-2 weeks for basic functionality
+
+**Dependencies:** None (standalone tool)
+
+---
+
 ## Future Goals