feat(find_model gui): toggle to overlay full SAM21 cluster (yellow)

Adds a 4th checkbox 'Punti SAM21 (gialli)' that draws the full SAM21
segmentation (input to both the plane partition and Open3DModel) on
the RGB result canvas. The backend now returns sam21_points_px and
n_sam21 alongside the existing accepted/excluded splits.

Useful as a sanity check: visualise what was actually segmented vs
what the plane filter kept/rejected vs where the STL got placed.

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

src/presa_mano/api/grasp_routes.py

@@ -306,6 +306,11 @@ def find_model(req: FindModelReq) -> dict[str, Any]:
     with sw.step("project_points"):
         accepted_px = project_camera_to_pixels(accepted_m, K, (W, H))
         excluded_px = project_camera_to_pixels(excluded_m, K, (W, H))
+        # Full SAM21 cluster (= input to the partition + Open3DModel search).
+        # Useful as a sanity-check overlay: shows everything segmented before
+        # any plane filtering. Equal to accepted ∪ excluded set-wise but kept
+        # as a separate field so the GUI can color it independently.
+        sam21_px = project_camera_to_pixels(seg.points_cv_m, K, (W, H))
 
     with sw.step("encode_rgb"):
         rgb_b64 = base64.b64encode(seg.rgb_jpeg).decode("ascii")
@@ -323,8 +328,10 @@ def find_model(req: FindModelReq) -> dict[str, Any]:
         "model_points_px":    model_px.tolist(),
         "accepted_points_px": accepted_px.tolist(),
         "excluded_points_px": excluded_px.tolist(),
+        "sam21_points_px":    sam21_px.tolist(),
         "n_accepted": int(accepted_m.shape[0]),
         "n_excluded": int(excluded_m.shape[0]),
+        "n_sam21": int(seg.points_cv_m.shape[0]),
         "plane_applied": bool(plane_info["plane_applied"]),
         "fit_ok": fit_ok,
         "fit_failed": fit_failed,