Agent-native version control for Lex (#128 tier-2).

The unit of writing is an [Operation] — a typed delta on the AST identified by (kind, payload, parents). Two agents producing the same logical change against the same parent state get the same [OpId], so the store can dedup automatically and surface "we agree" without a merge.

This crate is the foundation slice of #129. It defines the operation enum and content-addressed identity. Subsequent slices add: applying ops to a store state (#129 cont'd), the write-time type-check gate (#130), intent linkage (#131), attestations (#132), predicate branches (#133), and the programmatic merge API (#134).

Identity

[OpId] is the lowercase-hex SHA-256 of the canonical JSON form of (kind, payload, parents). The serializer is deterministic by construction (struct fields are emitted in declaration order; [EffectSet] is a BTreeSet; parents are sorted before hashing), so two independent runs producing the same logical operation produce byte-identical canonical bytes.

lex-store already uses SHA-256 (via the sha2 crate) for stage and signature identity, so we reuse that here for consistency and to avoid pulling in a second hash dependency. The issue text mentions Blake3; if that becomes load-bearing for performance we can swap with a one-line crate change since OpId is opaque.