feat(find_model): zero below-plane depth before SAM21 segmentation

Previously SAM21 ran on the raw aligned depth and frequently leaked the
mask onto the table around the object: ~88% of segmented points (2136
of 2440 in a sample run) were table pixels filtered away by the post
partition_by_plane step. Open3DModel got a noisy cluster mostly off
the object.

New pipeline: with a calibrated work plane, depth pixels whose 3D
back-projection sits below the plane are zeroed BEFORE SAM21 sees the
depth PNG. SAM21 cannot back-project zero-depth pixels, so only true
above-plane points end up in the cloud regardless of mask outline.

Same sample run after the change: 513 SAM21 -> 513 accepted, 0 excluded.

The post-partition still runs (now a no-op on a calibrated plane), so
the find_model response shape is unchanged.

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

src/presa_mano/perception/sam21_bridge.py

@@ -18,11 +18,44 @@ from ..clients import NuwaError, Sam21Error, Sam21Unavailable
 from ..timing import Stopwatch
 from .depth_align import align_depth_to_rgb
 from .depth_to_rgb import CameraRegistration
-from .work_plane import apply_below_plane_filter, get_work_plane
+from .work_plane import WorkPlane, apply_below_plane_filter, get_work_plane
 
 log = logging.getLogger(__name__)
 
 
+def zero_below_plane_depth(
+    depth_aligned_mm: np.ndarray,
+    intr_rgb: dict,
+    wp: WorkPlane,
+) -> tuple[np.ndarray, int]:
+    """Zero-out depth pixels whose 3D back-projection sits below the plane.
+
+    Per-pixel back-projection: P = K_rgb^-1 * [u,v,1] * depth(u,v) (mm).
+    Signed distance to the plane (in m): n . (P/1000) + offset.
+    Pixel is dropped iff `signed < -offset_mm/1000` — same threshold as
+    apply_below_plane_filter so the SAM21-then-partition path and the
+    depth-prefilter-then-SAM21 path agree.
+
+    Returns (filtered_depth_mm, n_zeroed).
+    """
+    H, W = depth_aligned_mm.shape
+    fx = float(intr_rgb["fx"]); fy = float(intr_rgb["fy"])
+    cx = float(intr_rgb["cx"]); cy = float(intr_rgb["cy"])
+    z_mm = depth_aligned_mm.astype(np.float64)
+    u = np.arange(W, dtype=np.float64)[None, :]
+    v = np.arange(H, dtype=np.float64)[:, None]
+    x_mm = (u - cx) * z_mm / fx
+    y_mm = (v - cy) * z_mm / fy
+    n = np.asarray(wp.normal, dtype=np.float64)
+    signed_m = (n[0] * x_mm + n[1] * y_mm + n[2] * z_mm) / 1000.0 + float(wp.offset)
+    threshold_m = -float(wp.offset_mm) / 1000.0
+    invalid = (z_mm > 0) & (signed_m < threshold_m)
+    n_zeroed = int(invalid.sum())
+    out = depth_aligned_mm.copy()
+    out[invalid] = 0
+    return out, n_zeroed
+
+
 class Sam21Fallback(Exception):
     """The SAM21 path could not produce a usable cloud. Callers should
     fall back to the non-SAM21 path (bbox ROI cloud)."""
@@ -41,9 +74,16 @@ class Sam21Segmentation:
 
 
 def segment_with_sam21(nuwa, sam21, bbox: tuple[int, int, int, int],
-                       sw: Stopwatch) -> Sam21Segmentation:
+                       sw: Stopwatch,
+                       pre_filter_plane: WorkPlane | None = None,
+                       ) -> Sam21Segmentation:
     """SAM21 puro: nuwa_grab + depth_align + segment, SENZA plane filter.
 
+    If `pre_filter_plane` is provided, depth pixels whose back-projection
+    sits below the plane are zeroed BEFORE SAM21 is called. This prevents
+    the segmentation from leaking onto the table around the object: SAM21
+    cannot back-project pixels with depth=0 into the cloud.
+
     Raises Sam21Fallback per failure semantiche (mask vuota, no depth, SAM21
     unreachable, decode fail). Re-raises HTTPException(502) per failure Nuwa.
     """
@@ -95,6 +135,11 @@ def segment_with_sam21(nuwa, sam21, bbox: tuple[int, int, int, int],
             }
             reg = CameraRegistration.from_nuwa(intr_full)
             depth_aligned = align_depth_to_rgb(depth_ir, reg)
+            if pre_filter_plane is not None:
+                depth_aligned, n_zeroed = zero_below_plane_depth(
+                    depth_aligned, intr_full["rgb"], pre_filter_plane)
+                log.info("pre_filter_plane: zeroed %d/%d depth pixels below plane",
+                         n_zeroed, depth_aligned.size)
             ok, depth_png_buf = cv2.imencode(".png", depth_aligned)
             if not ok:
                 log.warning("SAM21 path skipped: could not encode aligned depth as PNG")