TDD step 3: read_csv executes a file-source extraction — boundary-checked
path resolution via commonpath on canonical paths (rejects .. traversal,
absolute paths, symlink escapes, and prefix-collision siblings), utf-8-sig
decoding, streaming max_rows cap as a typed error (never silent
truncation), ragged-row rejection, and verbatim cells (escaping stays in
the §5 renderers). New SourceError in the typed hierarchy.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QeqhJpYQyghASjiJo5EhGg
