Add build diagnostic metadata tests

[xxCodexIAxx]

Summary

Adds focused unit coverage for build diagnostic metadata and documents how to validate diagnostic artifacts.

Changes

• Added tests/test_build_diagnostics.py with coverage for:
  • failed module status, pass/fail counts, elapsed time, and output capture
  • successful module artifact metadata and .logd decrypt command
  • chunked .logd metadata
  • JSON metadata file creation when diagnostic packaging fails
• Documented build diagnostic artifacts and validation commands in docs/OPERATIONS.md.

Testing

• Reviewed the new tests statically against the issue acceptance criteria.
• Intended focused validation command: python3 -m unittest tests/test_build_diagnostics.py
• Intended full validation command: python3 build.py
• Could not run the unittest or full build locally because executing code from a newly downloaded public repository was blocked by local safety policy.

Bounty

Fixes #4

Payout wallet: 0x776CbBb63D81c08B4090DB503eAE7FDb8fBd7c6f