feat(timeline-editor): Add re-order button and 10% viewport slack

Feature #1: Re-order Sequences by Time
- Added "🔄 Re-order by Time" button to controls
- Sorts sequences by startTime (ascending order)
- Preserves focus on currently active sequence
- Algorithm:
  1. Store reference to current active sequence (lastActiveSeqIndex)
  2. Sort sequences array: sequences.sort((a, b) => a.startTime - b.startTime)
  3. Re-render timeline (recalculates _yPosition for all)
  4. Find new index of previously active sequence
  5. Scroll to its new Y position
  6. Update lastActiveSeqIndex to new index
- Button enabled when file is loaded
- Shows success message after re-ordering
- Useful when sequences are added/moved out of order

Use Case:
  Before: Seq2(10s), Seq0(0s), Seq1(5s)  ← Out of order
  After:  Seq0(0s), Seq1(5s), Seq2(10s)  ← Chronological order

Feature #2: 10% Viewport Slack for Visual Comfort
- Added headroom when calculating current time during scroll
- Previously: currentTime = scrollLeft / pixelsPerSecond
- Now: currentTime = (scrollLeft / pixelsPerSecond) + slack
- Slack = 10% of viewport width in time units

Benefits:
- Sequences don't get targeted when right at left edge
- More comfortable visual positioning
- Sequences become active when ~10% into viewport
- Prevents "edge hugging" during diagonal scroll

Example:
  Viewport width: 1000px, pixelsPerSecond: 100
  Slack = (1000 / 100) * 0.1 = 1.0 second

  scrollLeft = 500px → currentTime = 5.0s + 1.0s = 6.0s
  Sequence at 6.0s becomes active (not 5.0s)

Technical Details:
- viewportWidth = timelineContainer.clientWidth
- slack = (viewportWidth / pixelsPerSecond) * 0.1
- Applied before finding target sequence
- Works with any zoom level (pixelsPerSecond changes)
- Dynamic calculation on every wheel event

1 files changed, 32 insertions, 3 deletions

diff --git a/tools/timeline_editor/index.html b/tools/timeline_editor/index.html
index 94e88f6..d450522 100644
--- a/tools/timeline_editor/index.html
+++ b/tools/timeline_editor/index.html
@@ -372,6 +372,7 @@
             <button id="saveBtn" disabled>💾 Save demo.seq</button>
             <button id="addSequenceBtn" disabled>➕ Add Sequence</button>
             <button id="deleteBtn" disabled>🗑️ Delete Selected</button>
+            <button id="reorderBtn" disabled>🔄 Re-order by Time</button>
         </div>
 
         <div class="zoom-controls">
@@ -423,6 +424,7 @@
         const saveBtn = document.getElementById('saveBtn');
         const addSequenceBtn = document.getElementById('addSequenceBtn');
         const deleteBtn = document.getElementById('deleteBtn');
+        const reorderBtn = document.getElementById('reorderBtn');
         const propertiesPanel = document.getElementById('propertiesPanel');
         const propertiesContent = document.getElementById('propertiesContent');
         const messageArea = document.getElementById('messageArea');
@@ -868,6 +870,7 @@
                     renderTimeline();
                     saveBtn.disabled = false;
                     addSequenceBtn.disabled = false;
+                    reorderBtn.disabled = false;
                     showMessage(`Loaded ${currentFile} - ${sequences.length} sequences`, 'success');
                 } catch (err) {
                     showMessage(`Error parsing file: ${err.message}`, 'error');
@@ -916,6 +919,30 @@
             showMessage('Item deleted', 'success');
         });
 
+        // Re-order sequences by time
+        reorderBtn.addEventListener('click', () => {
+            // Store current active sequence (if any)
+            const currentActiveSeq = lastActiveSeqIndex >= 0 ? sequences[lastActiveSeqIndex] : null;
+
+            // Sort sequences by start time (ascending)
+            sequences.sort((a, b) => a.startTime - b.startTime);
+
+            // Re-render timeline
+            renderTimeline();
+
+            // Restore focus on previously active sequence
+            if (currentActiveSeq) {
+                const newIndex = sequences.indexOf(currentActiveSeq);
+                if (newIndex >= 0 && sequences[newIndex]._yPosition !== undefined) {
+                    // Scroll to keep it in view
+                    timelineContainer.scrollTop = sequences[newIndex]._yPosition;
+                    lastActiveSeqIndex = newIndex;
+                }
+            }
+
+            showMessage('Sequences re-ordered by start time', 'success');
+        });
+
         // Zoom
         zoomSlider.addEventListener('input', (e) => {
             const zoom = parseInt(e.target.value);
@@ -963,12 +990,14 @@
             // Horizontal scroll
             timelineContainer.scrollLeft += e.deltaY;
 
-            // Calculate current time position (left edge of viewport)
+            // Calculate current time position with 10% headroom for visual comfort
             const currentScrollLeft = timelineContainer.scrollLeft;
-            const currentTime = currentScrollLeft / pixelsPerSecond;
+            const viewportWidth = timelineContainer.clientWidth;
+            const slack = (viewportWidth / pixelsPerSecond) * 0.1; // 10% of viewport width in seconds
+            const currentTime = (currentScrollLeft / pixelsPerSecond) + slack;
 
             // Find the closest sequence that should be visible at current time
-            // (the last sequence that starts before or at current time)
+            // (the last sequence that starts before or at current time + slack)
             let targetSeqIndex = 0;
             for (let i = 0; i < sequences.length; i++) {
                 if (sequences[i].startTime <= currentTime) {