fix(grasp): single-log SAM21 fallback + fail-fast on Nuwa errors

- All SAM21-path logging now lives in _points_via_sam21 at the right
  level (WARNING for operational, ERROR for bug codes / unexpected).
  _object_points_in_base no longer double-logs on fallback.
- Nuwa failures during SAM21 prep now raise HTTPException(502)
  immediately instead of running the futile bbox fallback (which would
  also fail because Nuwa is down).

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

src/presa_mano/api/grasp_routes.py

@@ -13,7 +13,9 @@ from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
 
 from ..calibration import CalibrationStore
-from ..clients import G1Unavailable, Sam21Error, Sam21Unavailable
+import httpx
+
+from ..clients import G1Unavailable, NuwaError, Sam21Error, Sam21Unavailable
 from ..config import get_config
 from ..geometry import transform_points
 from ..grasp import GraspPlanner, compute_axes
@@ -93,7 +95,7 @@ def _object_points_in_base(conf: float | None, class_name: str | None, sw: Stopw
             meta["n_points"] = int(len(pts_base))
             return pts_base, meta
         except _Sam21Fallback as e:
-            log.warning("SAM21 path failed, falling back to bbox: %s", e)
+            # Logging happens inside _points_via_sam21 with the right level.
             meta["sam21_fallback_reason"] = str(e)
 
     pts_base = _points_via_bbox(nuwa, T_base_cam, bbox, sw)
@@ -132,27 +134,37 @@ def _points_via_bbox(nuwa, T_base_cam: np.ndarray, bbox: tuple[int, int, int, in
 def _points_via_sam21(nuwa, bbox: tuple[int, int, int, int], sw: Stopwatch) -> np.ndarray:
     """SAM21 path: bbox → segmented point cloud in RGB-camera/OpenCV frame.
 
-    Returns points in **meters**. Raises _Sam21Fallback on any failure that
-    should trigger the bbox path; lets unexpected exceptions propagate as
-    _Sam21Fallback with repr(e).
+    Returns points in **meters**. Raises _Sam21Fallback on failures that should
+    trigger the bbox path. Re-raises HTTPException(502) for Nuwa failures
+    (the bbox path would fail too — better to fail fast). All SAM21-specific
+    logging happens here; the caller does not log on fallback.
     """
+    # Nuwa I/O: if it fails the bbox path will also fail, so fail fast with 502
+    # rather than logging a deceptive "falling back to bbox" warning.
     try:
         with sw.step("nuwa_grab"):
             rgb_jpeg = nuwa.rgb_jpeg()
             depth_png_in = nuwa.depth_png()
             intr_full = nuwa.intrinsics()
+    except (NuwaError, httpx.RequestError) as e:
+        raise HTTPException(status_code=502, detail=f"Nuwa unreachable: {e}")
+
     rgb_intr = intr_full.get("rgb")
     if not rgb_intr:
+        log.warning("SAM21 path skipped: nuwa intrinsics missing 'rgb' block")
         raise _Sam21Fallback("nuwa intrinsics missing 'rgb' block")
 
+    try:
         # JPEG → PNG. cv2.imdecode gives BGR; cv2.imencode(".png", bgr) writes
         # PNG bytes with standard R,G,B color order (libpng handles the swap),
-        # which SAM21's PIL-based decoder reads as RGB. Verified by inspection.
+        # which SAM21's PIL-based decoder reads as RGB.
         bgr = cv2.imdecode(np.frombuffer(rgb_jpeg, np.uint8), cv2.IMREAD_COLOR)
         if bgr is None:
+            log.warning("SAM21 path skipped: could not decode Nuwa RGB JPEG")
             raise _Sam21Fallback("could not decode Nuwa RGB JPEG")
         ok, rgb_png_buf = cv2.imencode(".png", bgr)
         if not ok:
+            log.warning("SAM21 path skipped: could not re-encode RGB as PNG")
             raise _Sam21Fallback("could not re-encode RGB as PNG")
         rgb_png = bytes(rgb_png_buf)
 
@@ -160,18 +172,23 @@ def _points_via_sam21(nuwa, bbox: tuple[int, int, int, int], sw: Stopwatch) -> n
             depth_ir = cv2.imdecode(np.frombuffer(depth_png_in, np.uint8),
                                     cv2.IMREAD_UNCHANGED)
             if depth_ir is None:
+                log.warning("SAM21 path skipped: could not decode Nuwa depth PNG")
                 raise _Sam21Fallback("could not decode Nuwa depth PNG")
             if depth_ir.dtype != np.uint16:
+                log.warning("SAM21 path skipped: depth PNG dtype %s (expected uint16)",
+                            depth_ir.dtype)
                 raise _Sam21Fallback(f"depth PNG dtype expected uint16, got {depth_ir.dtype}")
             reg = CameraRegistration.from_nuwa(intr_full)
             depth_aligned = align_depth_to_rgb(depth_ir, reg)
             ok, depth_png_buf = cv2.imencode(".png", depth_aligned)
             if not ok:
+                log.warning("SAM21 path skipped: could not encode aligned depth as PNG")
                 raise _Sam21Fallback("could not encode aligned depth as PNG")
             depth_png_out = bytes(depth_png_buf)
     except _Sam21Fallback:
         raise
     except Exception as e:
+        log.error("Unexpected SAM21 prep error: %r", e)
         raise _Sam21Fallback(f"prep failed: {e!r}") from e
 
     with sw.step("sam21"):
@@ -184,16 +201,20 @@ def _points_via_sam21(nuwa, bbox: tuple[int, int, int, int], sw: Stopwatch) -> n
                 depth_scale_mm=1.0,
             )
         except Sam21Unavailable as e:
+            log.warning("SAM21 unreachable, falling back to bbox: %s", e)
             raise _Sam21Fallback(f"unreachable: {e}") from e
         except Sam21Error as e:
             if e.code in _SAM21_BUG_CODES:
                 log.error("SAM21 rejected payload (%s): %s", e.code, e.message)
+            else:
+                log.warning("SAM21 returned %s: %s — falling back to bbox", e.code, e.message)
             raise _Sam21Fallback(f"{e.code}: {e.message}") from e
         except Exception as e:
             log.error("Unexpected SAM21 error: %r", e)
             raise _Sam21Fallback(f"unexpected: {e!r}") from e
 
     if pts_cv.shape[0] < 10:
+        log.warning("SAM21 returned %d points (<10), falling back to bbox", pts_cv.shape[0])
         raise _Sam21Fallback(f"too few points: {pts_cv.shape[0]}")
     return pts_cv