fix(gui): work_plane 3D view = camera POV + red = excluded points

- Camera at origin (CV frame) looking at +Z with screen-up = CV −Y, so
  the rendered scene matches the camera's vantage point. OrbitControls
  retargets to the plane centroid on fit so rotation feels natural.
- Simplify scatter coloring to two tones: red for filter-excluded points
  (signed_distance < −offset_mm/1000), cyan-white for everything kept.
  Matches the backend filter contract exactly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/work_plane.js

@@ -135,16 +135,26 @@ function startRgbLoop() {
   });
 })();
 
-// ---------------- Three.js 3D viewer ----------------
+// ---------------- Three.js 3D viewer — camera-POV ----------------
+//
+// CV camera frame: X right, Y down, Z forward.
+// We render the scene so the user sees it from the camera's vantage point:
+// camera at the origin, looking at +Z, with screen-up = CV -Y. OrbitControls
+// can rotate around the scene centroid once it's defined; before any fit the
+// view stays at camera-POV.
 const three = (function setupThree() {
   const canvas = $('wp3dCanvas');
   const scene = new THREE.Scene();
   const camera = new THREE.PerspectiveCamera(45, canvas.width / canvas.height, 0.01, 20);
-  camera.position.set(0.5, -0.5, -1.2);
-  camera.lookAt(0, 0, 1);
+  // Camera-POV: stand just behind the camera origin so OrbitControls has a
+  // non-zero distance to its target. Screen-up = CV -Y.
+  camera.up.set(0, -1, 0);
+  camera.position.set(0, 0, -0.001);
   const renderer = new THREE.WebGLRenderer({ canvas, antialias: true });
   renderer.setSize(canvas.width, canvas.height);
   const controls = new OrbitControls(camera, canvas);
+  controls.target.set(0, 0, 1);
+  controls.update();
   scene.add(new THREE.AxesHelper(0.1));
   const planeMesh = new THREE.Mesh(
     new THREE.PlaneGeometry(0.4, 0.4),
@@ -165,6 +175,9 @@ const three = (function setupThree() {
     // Align plane to its normal.
     planeMesh.lookAt(c[0] + n[0], c[1] + n[1], c[2] + n[2]);
     planeMesh.visible = true;
+    // Move the orbit target onto the plane centroid so rotation feels natural.
+    controls.target.set(c[0], c[1], c[2]);
+    controls.update();
   }
 
   function setScatter(pointsCv, plane, offsetMm) {
@@ -178,17 +191,18 @@ const three = (function setupThree() {
     const nz = plane ? plane.normal[2] : 0;
     const d = plane ? plane.offset : 0;
     const offsetM = (offsetMm || 0) / 1000.0;
-    const rmsM = plane ? (plane.fit_rms_mm / 1000.0) * 2.0 : 0.01;
+    // Two-tone colouring: filter-excluded points (signed < -offset_m) red,
+    // everything else cyan-white. This matches the backend filter contract
+    // (keep == signed >= -offset_m).
     for (let i = 0; i < n; i++) {
       const x = pointsCv[i][0], y = pointsCv[i][1], z = pointsCv[i][2];
       positions[3 * i] = x; positions[3 * i + 1] = y; positions[3 * i + 2] = z;
       const signed = plane ? (nx * x + ny * y + nz * z + d) : 0;
-      let r = 0.5, g = 0.5, b = 0.5;
-      if (plane) {
-        if (signed > offsetM)       { r = 1.0; g = 0.83; b = 0.31; }   // yellow: above plane+offset
-        else if (signed < -offsetM) { r = 0.93; g = 0.27; b = 0.27; }  // red: below plane-offset
-        else if (Math.abs(signed) <= rmsM) { r = 0.30; g = 0.78; b = 0.40; }  // green: inlier
-        else                        { r = 0.55; g = 0.55; b = 0.55; }  // grey: buffer
+      let r, g, b;
+      if (plane && signed < -offsetM) {
+        r = 0.93; g = 0.27; b = 0.27;        // red: excluded by below-plane filter
+      } else {
+        r = 0.75; g = 0.92; b = 1.00;        // cyan-white: kept
       }
       colors[3 * i] = r; colors[3 * i + 1] = g; colors[3 * i + 2] = b;
     }