Avatar works seated: only shoulders required, bust mode without hips

Sitting at a desk the hips are rarely visible, which made the avatar
never appear. The pelvis is now synthesized below the shoulders when
hidden (torso/head/arms as a bust; legs return when actually visible).
The output also reports diagnostics to the panel: pose tracker failed,
or avatar active with no body detected for 5s.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/interactive.js

@@ -1027,15 +1027,20 @@ class InteractiveSim {
     // tracked skeleton 1:1 — raise your arm and the robot raises its arm.
     if (mode === 'avatar') {
       const L = this.lmsRaw;
-      const ok = this.modelSim && L && L[11] && L[11].vis > 0.5 && L[12].vis > 0.5
-        && L[23].vis > 0.4 && L[24].vis > 0.4;
+      // Only the shoulders are required: seated at a desk the hips are often
+      // hidden, so the pelvis is synthesized and the robot shows as a bust.
+      const ok = this.modelSim && L && L[11] && L[11].vis > 0.5 && L[12].vis > 0.5;
       if (!ok) { this._drawField(7, e, audio, canvas, 0); return; }
       const S = 2.6;
       const W = (i) => [(L[i].x - 0.5)*S, (0.5 - L[i].y)*S, -(L[i].z || 0)*1.2];
       const mid = (p, q) => [(p[0]+q[0])/2, (p[1]+q[1])/2, (p[2]+q[2])/2];
-      const sh1 = W(11), sh2 = W(12), hp1 = W(23), hp2 = W(24);
-      const neck = mid(sh1, sh2), pelvis = mid(hp1, hp2);
+      const sh1 = W(11), sh2 = W(12);
+      const neck = mid(sh1, sh2);
       const shw = Math.max(0.15, Math.hypot(sh1[0]-sh2[0], sh1[1]-sh2[1], sh1[2]-sh2[2]));
+      const hipsVis = L[23].vis > 0.4 && L[24].vis > 0.4;
+      const hp1 = hipsVis ? W(23) : [neck[0]-shw*0.28, neck[1]-shw*1.25, neck[2]];
+      const hp2 = hipsVis ? W(24) : [neck[0]+shw*0.28, neck[1]-shw*1.25, neck[2]];
+      const pelvis = mid(hp1, hp2);
       const t = shw;
       const metal = [0.60, 0.64, 0.72], dark = [0.28, 0.30, 0.36];
       const acc = e.colorB || [0.2, 1, 1];
@@ -1063,9 +1068,9 @@ class InteractiveSim {
       };
       arm(sh1, 13, 15); arm(sh2, 14, 16);
       ball(sh1, t*0.16, dark); ball(sh2, t*0.16, dark);
-      // legs with feet
+      // legs with feet (only when actually visible: seated = bust only)
       const leg = (h, knI, anI) => {
-        if (L[knI].vis < 0.4 || L[anI].vis < 0.4) return;
+        if (!hipsVis || L[knI].vis < 0.4 || L[anI].vis < 0.4) return;
         const kn = W(knI), an = W(anI);
         limb(h, kn, t*0.16, metal);
         limb(kn, an, t*0.13, metal);