Fix waveform crossfade hatch + click hit-detection to use contact edges

- Crossfade hatch was centered on segments[i+1][0] (old algo edge);
now centred on contactEdges[i] = (seg[i][1]+seg[i+1][0])/2,
which is where _stitch_wavs actually applies the blend.
- Click hit-detection used segments[idx+1][0] as boundary; now uses
contactEdges[idx] to match the visual coloring exactly.

app.py

@@ -2097,12 +2097,12 @@ def _build_waveform_html(audio_path: str, segments: list, slot_id: str,
     }});
 
     // ── Crossfade overlap indicators ──
-    // The color boundary is at segments[i+1][0] (= seg_i.end - crossfadeSec).
-    // We centre the hatch on that edge: half the crossfade on each color side.
+    // The crossfade is applied ±crossfadeSec/2 around each contact edge.
+    // contactEdges[i] = midpoint of overlap between seg i and seg i+1.
     if (crossfadeSec > 0 && segments.length > 1) {{
       for (let i = 0; i < segments.length - 1; i++) {{
-        // Color edge = segments[i+1][0], hatch spans half on each side
-        const edgeT       = segments[i+1][0];
+        // Hatch spans crossfadeSec centred on the contact edge
+        const edgeT        = contactEdges[i];
         const overlapStart = edgeT - crossfadeSec / 2;
         const overlapEnd   = edgeT + crossfadeSec / 2;
         const xL = (overlapStart / duration) * W;
@@ -2129,12 +2129,13 @@ def _build_waveform_html(audio_path: str, segments: list, slot_id: str,
       const r=cv.getBoundingClientRect();
       const xRel=(e.clientX-r.left)/r.width;
       const tClick=xRel*duration;
-      // Pick the segment whose unique (non-overlapping) region contains the click.
-      // Each segment owns [seg[0], nextSeg[0]) visually; last segment owns [seg[0], seg[1]].
+      // Pick the segment by contact-edge boundaries — matches the visual coloring.
+      // Seg 0 owns [0, contactEdges[0]), seg N-1 owns [contactEdges[N-2], duration).
       let hit=-1;
       segments.forEach(function(seg,idx){{
-        const uniqueEnd = idx + 1 < segments.length ? segments[idx+1][0] : seg[1];
-        if (tClick >= seg[0] && tClick < uniqueEnd) hit = idx;
+        const visStart = idx === 0 ? 0 : contactEdges[idx-1];
+        const visEnd   = idx === segments.length - 1 ? duration : contactEdges[idx];
+        if (tClick >= visStart && tClick < visEnd) hit = idx;
       }});
       console.log('[wf click] tClick='+tClick.toFixed(2)+' hit='+hit+' audioDuration='+audioDuration+' segments='+JSON.stringify(segments));
       if (hit>=0) showPopup(hit, e.clientX, e.clientY);