Pure module computing deterministic 16-char SHA-256 prefix for annotation set.
Canonicalization: sort by id, fixed field order (id|target_artifact|target_anchor|intent|comment|timestamp), \n-join, sha256, take first 16 hex.
Brief SC4 specifies sha256-prefix; research-05 said sha1 — brief wins per Hard Rule "Brief-driven".
6 tests pass: empty digest, order-independence, intent-sensitivity, format invariant, golden value, undefined-vs-empty equivalence.
