Implement the frontmatter submodule:
- detect_delimiters scans for a leading YAML (---) or TOML (+++) block at
byte 0. Strict per §0 Q9: no leading whitespace / BOM, no chars on the
delimiter line. Closing must be its own line. Unterminated → no FM.
- parse_raw deserializes into RawFrontmatter, a serde-flatten struct that
catches unknown keys into a serde_json::Map for verbatim preservation
in metadata.user.
- derive_metadata implements the §0 Q9 fallback chain:
title → frontmatter | BodyHints.first_h1 | (filename: caller)
aliases/tags→ frontmatter | []
lang → frontmatter | lingua autodetect on first 4 KB | hints
| "und"
created_at → frontmatter (RFC 3339, normalized to UTC) | fs_ctime
updated_at → frontmatter | fs_mtime
source_type → frontmatter | "markdown"
trust_level → frontmatter | "primary"
id → user_id_alias only — never a doc_id factor (§4.2)
- Non-UTC offsets are normalized to UTC; the original string is preserved
in user.original_timestamps[field] per §0 Q9.
- Warnings are emitted for: malformed YAML/TOML, unknown enum values,
malformed timestamps. Unknown keys are silent.
- lingua detector is cached in a OnceLock — first build is heavy.
- 15 unit tests cover every row of the derive table + delimiter edge
cases + an explicit pin that `id:` does not feed id_for_doc.
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