fix: handle Qwen and Gemini grounding coordinate orders

[Anionex]

Summary

• select Gemini yxyx versus Qwen-family xyxy grounding coordinates by model family, with VISION_BOX_ORDER override support
• reject truncated bounding-box JSON instead of returning partial matches
• make detect target construction idempotent
• document the provider coordinate conventions in the vision-tools Skill

Verification

• python3 tests/test_ground.py
• python3 tests/test_detect.py
• python3 -m py_compile ground.py detect.py
• live Qwen3.6 grounding on a 1200x675 image produced a horizontal top-title box after xyxy parsing

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