CrdtTree Contract (#lzcrdttree)

CrdtTree is the shared lossless document-CRDT seam used by snapshots, relay canonicals, and document-op replication. An implementation exposes:

• version_vector() — a compact distributed frontier;
• delta_since(frontier) — every operation the frontier has not observed;
• apply_delta(delta) — an identity-preserving, idempotent fold;
• text() / value() — the visible lossless projection; and
• merge_from(other) — the state join.

The join and delta fold are commutative, associative, and idempotent. A full snapshot is exactly delta_since(empty_frontier); applying it to an empty replica must preserve operation identities, not reparse the visible text and mint a new lineage. Applying delta_since(version_vector()) is a no-op.

The compute fixture at conformance/crdt-tree/algebra.json pins merge order independence, idempotence, snapshot equivalence, and incremental round-trip behavior. The Lean model in LazilyFormal.Replication pins the three join laws.