Summary

• Count start and end markers before update_file_section modifies a file.
• Return a clear error when a file contains only one side of the managed marker pair.
• Add BATS coverage for start-only and end-only corrupted sections.

Validation

• bats lib/bash/file/tests/lib_file.bats
• env -u BASE_HOME HOME=/private/tmp/base-review-home BASE_TEST_PYTHON=/Users/rameshhp/.base.d/base/.venv/bin/python bin/base-test
• git diff --check

Closes #330