feat(06-02): add segment overlay canvas rendering and click interaction

- Create lightsync/frontend/timeline/segments.js with SegmentOverlay class
- Integrate SegmentOverlay into timeline.js render pipeline (between row banding and waveform)
- Add loadSegments method to TimelineCanvas
- Wire loadSegments call in app.js loadedmetadata handler

lightsync/frontend/timeline/segments.js

@@ -0,0 +1,122 @@
+// SegmentOverlay — draws structural section bands on the timeline canvas (Phase 6)
+// Bands are translucent, drawn below waveform. Clicking selects a section.
+
+const BAND_COLORS = [
+    'rgba(0,255,255,0.15)',    // --accent cyan variant
+    'rgba(180,0,255,0.15)',    // purple complement
+    'rgba(0,200,200,0.12)',    // darker cyan variant
+];
+
+class SegmentOverlay {
+    constructor() {
+        this.segments = [];       // [{start, end, label}, ...]
+        this.selectedIndex = -1;  // -1 = no selection
+        this.loading = false;
+        this.error = null;
+    }
+
+    /**
+     * Load segments from the API.
+     * @param {string} audioPath — absolute path to audio file (e.g. /app/shows/file.mp3)
+     */
+    async load(audioPath) {
+        this.loading = true;
+        this.error = null;
+        this.segments = [];
+        this.selectedIndex = -1;
+        try {
+            const res = await fetch(`/api/audio/segments?path=${encodeURIComponent(audioPath)}`);
+            if (!res.ok) throw new Error(`HTTP ${res.status}`);
+            const data = await res.json();
+            this.segments = data.segments || [];
+        } catch (e) {
+            this.error = `SEGMENTATION FAILED: ${e.message}`;
+            console.warn('[segments]', e);
+        } finally {
+            this.loading = false;
+        }
+    }
+
+    /**
+     * Get the currently selected segment, or null.
+     * @returns {{start: number, end: number, label: string} | null}
+     */
+    getSelected() {
+        if (this.selectedIndex >= 0 && this.selectedIndex < this.segments.length) {
+            return this.segments[this.selectedIndex];
+        }
+        return null;
+    }
+
+    /**
+     * Handle a click at a given time position. Selects the segment containing that time,
+     * or deselects if clicking outside any segment.
+     * @param {number} time — time in seconds
+     * @returns {boolean} — true if selection changed
+     */
+    handleClick(time) {
+        const prev = this.selectedIndex;
+        this.selectedIndex = -1;
+        for (let i = 0; i < this.segments.length; i++) {
+            if (time >= this.segments[i].start && time < this.segments[i].end) {
+                this.selectedIndex = i;
+                break;
+            }
+        }
+        return this.selectedIndex !== prev;
+    }
+
+    /**
+     * Draw segment bands on the canvas context.
+     * Must be called BETWEEN track row banding and waveform in the render pipeline.
+     *
+     * @param {CanvasRenderingContext2D} ctx
+     * @param {number} W — canvas logical width
+     * @param {number} H — canvas logical height
+     * @param {number} trackCount — number of tracks
+     * @param {Function} timeToX — converts time (seconds) to canvas X coordinate
+     */
+    draw(ctx, W, H, trackCount, timeToX) {
+        if (this.segments.length === 0) return;
+
+        const totalH = trackCount * 48;  // TRACK_HEIGHT
+        const topY = 24;                 // TIME_AXIS_HEIGHT
+        const headerW = 140;             // HEADER_WIDTH
+
+        for (let i = 0; i < this.segments.length; i++) {
+            const seg = this.segments[i];
+            const x1 = timeToX(seg.start);
+            const x2 = timeToX(seg.end);
+
+            // Skip segments entirely off-screen
+            if (x2 < headerW || x1 > W) continue;
+
+            // Clamp to visible area
+            const drawX1 = Math.max(x1, headerW);
+            const drawX2 = Math.min(x2, W);
+            const drawW = drawX2 - drawX1;
+
+            const isSelected = this.selectedIndex === i;
+
+            // D-09: Translucent colored band, full waveform height, alternating colors
+            ctx.fillStyle = BAND_COLORS[i % BAND_COLORS.length];
+            ctx.fillRect(drawX1, topY, drawW, totalH);
+
+            // D-11: Selected section gets accent-color border highlight
+            if (isSelected) {
+                ctx.strokeStyle = 'rgba(0,255,255,1.0)';
+                ctx.lineWidth = 1;
+                ctx.strokeRect(drawX1, topY, drawW, totalH);
+            }
+
+            // D-10: Label — small monospace uppercase at top-left of band
+            if (drawW > 30) {
+                ctx.fillStyle = 'rgba(0,255,255,0.6)';
+                ctx.font = '9px monospace';
+                ctx.fillText(seg.label, Math.max(x1 + 3, headerW + 2), topY + 10);
+            }
+        }
+    }
+}
+
+export { SegmentOverlay };

lightsync/frontend/timeline/timeline.js

@@ -5,6 +5,7 @@
 import { CommandHistory } from './history.js';
 import { PlaceBlockCommand, MoveBlockCommand, ResizeBlockCommand, DeleteBlockCommand, setCurrentShowId } from './commands.js';
 import { BeatGrid } from './beats.js';
+import { SegmentOverlay } from './segments.js';
 
 const PIXELS_PER_SECOND = 100;  // default, adjustable via zoom
 const TRACK_HEIGHT = 48;
@@ -26,6 +27,7 @@ class TimelineCanvas {
         this.waveformPeaks = [];
         this.audioDuration = 0;
         this.beatGrid = new BeatGrid();
+        this.segmentOverlay = new SegmentOverlay();
         this.selectedBlock = null;
         this._dragState = null;
         this._onSelectBlock = null;  // callback(cue) for inspector
@@ -120,6 +122,11 @@ class TimelineCanvas {
             ctx.fillRect(HEADER_WIDTH, y, W - HEADER_WIDTH, TRACK_HEIGHT);
         });
 
+        // 2.5 Segment overlay bands (drawn below waveform)
+        if (this.segmentOverlay.segments.length > 0) {
+            this.segmentOverlay.draw(ctx, W, H, this.tracks.length, (t) => this.timeToX(t));
+        }
+
         // 3. Waveform background
         if (this.waveformPeaks.length > 0 && this.audioDuration > 0) {
             const totalH = this.tracks.length * TRACK_HEIGHT;
@@ -406,9 +413,11 @@ class TimelineCanvas {
             const hit = this.hitTest(mx, my);
 
             if (!hit) {
-                // Clicked on empty canvas — deselect
+                // Clicked on empty canvas — deselect block, update segment selection
                 this.selectedBlock = null;
                 if (this._onSelectBlock) this._onSelectBlock(null);
+                const clickTime = this.xToTime(mx);
+                this.segmentOverlay.handleClick(clickTime);
                 return;
             }
 
@@ -622,6 +631,10 @@ class TimelineCanvas {
     async loadBeats(audioPath) {
         await this.beatGrid.load(audioPath);
     }
+
+    async loadSegments(audioPath) {
+        await this.segmentOverlay.load(audioPath);
+    }
 }
 
 export { TimelineCanvas };