Function normalize_document 

pub fn normalize_document(doc: Document) -> Result<Document, ParseError>

Primary entry point for normalizing a crate::document::Document after parsing.

Per-card normalization:

1. Payload field names → Unicode NFC.
2. Card body → bidi-stripped + HTML comment fence repair (spec §7). YAML field values pass through verbatim.

Idempotent — calling multiple times produces the same result.