feat(find_model gui): toggle to overlay work plane as a violet polygon

The find_model response now returns plane_polygon_px (4 corners of a
30 cm square anchored at the work plane centroid, projected to RGB
pixels via the same K used for the SAM21 cloud). The GUI gains a
'Piano (viola)' checkbox that draws the polygon as a translucent
violet quad with an outline, helping the operator see whether the
plane sits where expected relative to the object.

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

src/presa_mano/api/grasp_routes.py

@@ -316,6 +316,34 @@ def find_model(req: FindModelReq) -> dict[str, Any]:
         # as a separate field so the GUI can color it independently.
         sam21_px = project_camera_to_pixels(seg.points_cv_m, K, (W, H))
 
+    # 6. Project the work plane as a 30cm square anchored at its centroid,
+    #    so the GUI can overlay it on the RGB image.
+    plane_polygon_px: list[list[int]] = []
+    wp = get_work_plane()
+    if wp is not None:
+        with sw.step("project_plane"):
+            n_vec = np.asarray(wp.normal, dtype=np.float64)
+            c_vec = np.asarray(wp.centroid, dtype=np.float64)
+            # Pick an arbitrary in-plane vector orthogonal to n.
+            ref = np.array([1.0, 0.0, 0.0]) if abs(n_vec[0]) < 0.9 else np.array([0.0, 1.0, 0.0])
+            u_vec = ref - (ref @ n_vec) * n_vec
+            u_vec /= np.linalg.norm(u_vec) or 1.0
+            v_vec = np.cross(n_vec, u_vec)
+            half = 0.15  # 30 cm square
+            corners = np.stack([
+                c_vec + half * u_vec + half * v_vec,
+                c_vec - half * u_vec + half * v_vec,
+                c_vec - half * u_vec - half * v_vec,
+                c_vec + half * u_vec - half * v_vec,
+            ])
+            # Project each corner manually: project_camera_to_pixels drops
+            # off-frame points which would break the polygon, so allow them.
+            zs = corners[:, 2]
+            if (zs > 0).all():
+                proj = (K @ corners.T).T
+                uv = proj[:, :2] / proj[:, 2:3]
+                plane_polygon_px = np.round(uv).astype(int).tolist()
+
     with sw.step("encode_rgb"):
         rgb_b64 = base64.b64encode(seg.rgb_jpeg).decode("ascii")
 
@@ -333,6 +361,7 @@ def find_model(req: FindModelReq) -> dict[str, Any]:
         "accepted_points_px": accepted_px.tolist(),
         "excluded_points_px": excluded_px.tolist(),
         "sam21_points_px":    sam21_px.tolist(),
+        "plane_polygon_px":   plane_polygon_px,
         "n_accepted": int(accepted_m.shape[0]),
         "n_excluded": int(excluded_m.shape[0]),
         "n_sam21": int(seg.points_cv_m.shape[0]),