docs: update CLAUDE.md for SAM21 + fix httpx import order

- Note that tests now exist and run with uv run pytest.
- Add SAM21 to the upstream services table.
- Add sam21() to the _deps factory list.
- Add a convention bullet about the SAM21 vs bbox frame asymmetry so
  future contributors don't add nuwa_to_cv_frame to the SAM21 branch.
- Move 'import httpx' into the third-party imports block.

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

src/presa_mano/api/grasp_routes.py

@@ -6,6 +6,7 @@ import logging
 from typing import Any, Optional
 
 import cv2
+import httpx
 import numpy as np
 import time
 from dataclasses import asdict
@@ -13,8 +14,6 @@ from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
 
 from ..calibration import CalibrationStore
-import httpx
-
 from ..clients import G1Unavailable, NuwaError, Sam21Error, Sam21Unavailable
 from ..config import get_config
 from ..geometry import transform_points