.mailmap maps author/committer identities to canonical ones. Blame, log, contributor lists all look nicer with mailmap applied.

Scope

• Mailmap.load(repo, tree_sha) → parsed mapping.
• Mailmap.canonicalize(mailmap, name, email) → {canonical_name, canonical_email}.
• Support all four mailmap line forms (name+email, email, name, full replacement).

Acceptance

• Matches git shortlog --use-mailmap attribution on a fixture with 4 aliased identities.