test(drawio): isolate generated artifact version provenance

[Alpha9463]

Summary

Version-only package releases no longer require regenerating committed gallery or Draw.io artifacts.

• replaces the full-example Draw.io freshness check with a narrow regression test that proves a version change affects only mxfile/@agent provenance;
• adds a shared helper for normalising that provenance during comparison;
• clarifies when gallery and Draw.io outputs must be regenerated in contributor, generator, and fixture documentation;
• simplifies the Draw.io sample generator’s documentation without changing its output behaviour.

Verification

• python3 -m pytest
  • 11,459 passed, 190 skipped
• ruff check .
• ruff format --check tests
• python3 -m mypy pandid
• python3 -m pyright pandid
  • 0 errors; one existing __all__ warning
• git diff --check

Closes #546