Normalize RECORD paths when writing

[ahmojo]

Summary

Fixes RECORD writing so Windows-style separators are normalized to forward slashes regardless of the platform doing the write.

What changed

• Updated RecordEntry.to_row() to normalize the output path with PureWindowsPath(...).as_posix().
• Added a focused regression test that simulates a non-Windows writer receiving a backslash path.
• No new dependencies.

Why

RecordEntry.to_row() previously only replaced backslashes when os.sep == \. That meant a RecordEntry containing a Windows-style path could still write backslashes when serialized on another platform. RECORD paths should use / consistently.

Tests

• python -m pytest tests/test_records.py::TestRecordEntry::test_string_representation_normalizes_backslash_path -q
• python -m pytest tests/test_records.py -q
• python -m ruff check src\installer\records.py tests\test_records.py
• python -m ruff format --check src\installer\records.py tests\test_records.py
• python -m nox -s test-3.14
• python -m pre_commit run --files src\installer\records.py tests\test_records.py

Closes #158