feat(perception): partition_by_plane returns accepted + excluded sets

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

src/presa_mano/perception/work_plane.py

@@ -122,25 +122,35 @@ def fit_plane_svd(points_xyz: np.ndarray
     return normal, offset, c, rms_mm
 
 
-def apply_below_plane_filter(
+def partition_by_plane(
     pts_cv: np.ndarray,
     wp: WorkPlane | None,
-) -> tuple[np.ndarray, dict]:
-    """Drop points more than `offset_mm` below the plane.
+) -> tuple[np.ndarray, np.ndarray, dict]:
+    """Split points into (accepted_above_plane, excluded_below_plane).
 
-    Returns `(filtered_pts, info)`. `info` always carries:
-        plane_applied (bool): true iff a plane was provided AND there were
-            points to filter
-        n_points_before_plane (int): len(pts_cv) on entry
+    Same threshold as `apply_below_plane_filter`: keep iff
+    `signed >= -offset_mm/1000`. When `wp is None` or `pts_cv` is empty,
+    `excluded` is empty and `plane_applied=False`.
     """
     info = {"plane_applied": False,
             "n_points_before_plane": int(len(pts_cv))}
+    empty = np.empty((0, 3), dtype=pts_cv.dtype if pts_cv.size else np.float64)
     if wp is None or len(pts_cv) == 0:
-        return pts_cv, info
+        return pts_cv, empty, info
     n_vec = np.asarray(wp.normal, dtype=np.float64)
     d_off = float(wp.offset)
     offset_m = float(wp.offset_mm) / 1000.0
     signed = pts_cv @ n_vec + d_off
     keep = signed >= -offset_m
     info["plane_applied"] = True
-    return pts_cv[keep], info
+    return pts_cv[keep], pts_cv[~keep], info
+
+
+def apply_below_plane_filter(
+    pts_cv: np.ndarray,
+    wp: WorkPlane | None,
+) -> tuple[np.ndarray, dict]:
+    """Thin wrapper around partition_by_plane that drops the excluded set.
+    Kept for backwards compatibility with existing callers."""
+    accepted, _excluded, info = partition_by_plane(pts_cv, wp)
+    return accepted, info