Method annotate_delta

Set a local annotation (e.g. a commit message) on an existing retired delta and re-persist it. Unlike the per-interaction marker written inline at retire, this targets an already-written delta, so the whole history file is rewritten in topological order. O(history) — fine for occasional user labeling, not for per-interaction marking (which uses the inline path). No-op if `rev` is unknown.

(&mut self, rev: Rev, key: &str, value: serde_json::Value)