[NANDAHack] Improve memory layer: LWW-Register CRDT plugin with convergence validators

Description

The pitch.

Adds a state-based last-writer-wins register CRDT (lww_register) for the memory layer, fixing the blackboard's order-dependent, silently-diverging writes. Merge is a true semilattice join over (lamport, node, payload), so it is commutative, associative, and idempotent: replicas that have seen the same writes converge to byte-identical state regardless of delivery order, duplication, or loss.

Contents:
- LwwRegisterMemory plugin implementing the full Memory protocol plus an export/merge replication channel; registered in _BUILTINS and as a pyproject entry point under nest.plugins.memory.
- validate_crdt_convergence adversarial validator (fails on blackboard, passes on lww_register) plus memory_concurrent_writers trace validators.
- memory_concurrent_writers builtin scenario and YAML: 8 gossiping replicas converge under 10% message drop; deterministic across seeds 42, 7, 1337.
- 33 tests: protocol conformance, replication, Hypothesis property tests for the CRDT laws and convergence under arbitrary delivery order, malformed input handling, registry wiring, and an end-to-end scenario run.
- Memory layer docs updated.