Function validate_citation 

pub fn validate_citation(c: &Citation) -> Result<()>

v0.7.0 Form 4 (issue #757) — validate a Citation envelope.

Required invariants:

• uri is non-empty after trim and starts with one of the typed schemes accepted by validate_source_uri (mirror semantics — citation URIs and source URIs share the same form).
• accessed_at parses as RFC3339.
• hash (when present) is exactly 64 lowercase hex characters (SHA-256 digest).
• span (when present) satisfies validate_source_span.

Errors

Returns the first invariant failure encountered.