[codex] Handle empty files in partial port audit

[Vilin97]

What changed

• Skips zero-LOC upstream files when computing matched-file truncation ratios in the partial-port audit.
• Adds a regression test covering empty matched upstream files.

Why

Running the audit against import/hopfieldnet exposed a ZeroDivisionError when an upstream file had zero non-comment LOC. That made the advisory audit crash instead of reporting the actual partial-port signal.

Validation

• cd python && uv run --group test pytest tests/test_partial_port_audit.py
• cd python && uv run ruff check lean_pool/partial_port_audit.py tests/test_partial_port_audit.py
• cd python && uv run python -m lean_pool.partial_port_audit --repo .. --base-ref origin/main --head-ref import/hopfieldnet --json

[Copilot]

Pull request overview

Fixes a ZeroDivisionError in the partial-port audit when an upstream Lean file has zero non-comment LOC by skipping such files during the matched-file truncation ratio computation, and adds a regression test.

Changes:

• Skip zero-LOC upstream files in evaluate_stats truncation loop.
• Add regression test for empty matched upstream files.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/lean_pool/partial_port_audit.py	Guards the truncation-ratio loop against zero-LOC upstream files.
python/tests/test_partial_port_audit.py	Adds regression test verifying empty upstream files do not cause divide-by-zero.