Enforce a 500-line Python file-length ratchet in CI #111

Summary

Add a dependency-free CI check for the Python file-length policy recommended in docs/investigations/file-length-limit.md.

At the measured develop commit 902c759, 14 Python files exceeded 500 physical lines. Use a ratchet so existing files may shrink without requiring an immediate repository-wide split.

Scope

• Add a standard-library checker and a checked-in baseline for the current over-limit files.
• Count physical lines, including blank lines, comments, docstrings, and lines inside strings.
• Check tracked Python files under
  itrostack/, ests/, examples/, and scripts/.
• Enforce a 500-line maximum for new files and files at or below 500 lines.
• For existing offenders, fail if a file grows above its baseline; lower the baseline as the file shrinks.
• Count embedded templates in Python files. Exclude non-Python assets, vendored dependencies, and generated output.
• Run the check in the existing GitHub Actions workflow and report path, count, and allowed count.
• Create or update CONTRIBUTING.md with the definition, scope, exemptions, and baseline update process.

Acceptance criteria

• The checker has focused coverage for a new file over 500, a file exactly at 500, baseline growth, and a file below its baseline.
• CI runs the check across its supported Python matrix.
• The baseline and policy documentation agree with the checker behavior.
• No source files are split as part of this issue.

Out of scope: changing source behavior, splitting files, or replacing physical lines with logical SLOC.