Package extractor dispatches non-Markdown files to format-specific
extractors, each returning a *parser.ParseResult identical in shape to the
Markdown pipeline output.
Extract dispatches to the correct format extractor by file extension.
absPath is the absolute on-disk path; relPath is the path relative to the
project root (used as the canonical node ID prefix). src is the raw file
bytes; hash is the pre-computed SHA-256 hex string.