M1: Reword the FrontmatterSpan doc-comment from "technically meant to be
crate-internal" to a forward-looking note about P1-3 / P1-4 callers using
bytes[span.end..] for body slicing.
M3: Add an explicit `# Errors` section to parse_frontmatter's rustdoc.
The current implementation never returns Err — all recoverable problems
are downgraded to warnings — but the Result is kept on the signature so
future hard-fail conditions can be added without breaking callers.
M4: Mention serde_yml in the library-choice rationale alongside
serde_yaml_ng, with a one-line note on why _ng was preferred (stricter
adherence to original serde_yaml semantics around null / tagged enums).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
altair823
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