examples: add self-contained RF-DETR demo

[abhinavsriva]

Summary

Add --demo to the RF-DETR example so users can try TraceML without
preparing a dataset or creating a Roboflow account.

The example generates temporary COCO sample data, preserves the existing
real-dataset path, and documents the first-use pretrained weights download.

Validation

• 33 focused tests passed.
• CPU demo completed with released TraceML 0.5.0: 16 recorded steps and saved summaries.
• Formatting, lint, and strict documentation build passed.
• CUDA verification remains manual.

[coderabbitai]

Note

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⚙️ Run configuration

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• Run ID: 14ac61a4-00e8-401d-addf-38c4a96551f1

📥 Commits

Reviewing files that changed from the base of the PR and between 663d52e and 4a34aa8.

📒 Files selected for processing (4)

• docs/user_guide/integrations/rfdetr.md
• examples/README.md
• examples/integrations/rfdetr_minimal.py
• tests/integrations/test_rfdetr_example.py

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