Show inter-marker spread in dev HUD (updateDbg takes spread)

public/js/exhibit.js

@@ -43,7 +43,7 @@ let clockOffset = 0; // serverNow - clientNow
 const _dbgEuler = new THREE.Euler();
 const _dbgV = new THREE.Vector3();
 let dbgEl = null;
-function updateDbg(fusedQuat, markerCount) {
+function updateDbg(fusedQuat, markerCount, spread) {
   if (!dbgEl) return;
   const deg = r => (r * 180 / Math.PI).toFixed(0).padStart(4);
   // raw fused (CV-frame) orientation as YXZ euler
@@ -53,9 +53,10 @@ function updateDbg(fusedQuat, markerCount) {
   const view = _dbgV.set(0, 0, -1).applyQuaternion(camera3.quaternion).clone();
   const up = _dbgV.set(0, 1, 0).applyQuaternion(camera3.quaternion).clone();
   const v3 = v => `(${v.x.toFixed(2)},${v.y.toFixed(2)},${v.z.toFixed(2)})`;
+  const spreadLine = markerCount > 1 ? `  spread ${spread.toFixed(1)}cm` : '';
   dbgEl.textContent =
     `rot ${getRotMode()}  cam ${camMode}   (tap to cycle)\n` +
-    `markers ${markerCount}\n${rawLine}\nview ${v3(view)}\nup   ${v3(up)}\n` +
+    `markers ${markerCount}${spreadLine}\n${rawLine}\nview ${v3(view)}\nup   ${v3(up)}\n` +
     `pos  ${v3(camera3.position)}`;
 }
 
@@ -268,7 +269,7 @@ function loop(t) {
         // pose.js already conjugated rotation into the three.js frame; apply any
         // residual camera-frame correction (switchable via overlay)
         camera3.quaternion.copy(fused.quaternion).multiply(CAM_CORR[camMode]);
-        updateDbg(fused.quaternion, fused.markerCount);
+        updateDbg(fused.quaternion, fused.markerCount, fused.spread);
       }
     }
   }