Prevent CLI output from overwriting input

[Ray0907]

Summary

• reject --output paths that refer to the input file before conversion
• compare both normalized paths and file identity to cover hard links and symlinks
• add regression tests that verify the source bytes remain unchanged

Root cause

The CLI converted the document and passed --output directly to writeFile() without
checking whether it referred to the input. Using the same path, or an alias of it,
therefore replaced the source document with generated Markdown.

Impact

Invalid input/output combinations now fail with exit code 2 and leave the source file
untouched.

Validation

• npm test (15 passed)
• cargo test --locked (217 passed, 1 ignored)
• direct-path and hard-link regression tests
• symlink smoke test with matching source SHA-256 before and after rejection

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