Additional CRDTs: LWWMap, ORMap, MVRegister, GCounterMap

[pathosDev]

Follow-up to #37. The starter set (GCounter, PNCounter, GSet, ORSet, LWWRegister) covers the canonical scenarios. Akka's `ddata` ships several more that pull their weight in real workloads — same-shape merge functions, just composed differently.

To add:

• LWWMap<K, V> — map of LWWRegisters; each value carries its own `(timestamp, replicaId)`. Pattern: per-key feature flags, per-user settings.
• ORMap<K, V extends Crdt> — map of CRDTs; per-key add-wins semantics. Pattern: shopping carts where each cart is itself an ORSet of items.
• MVRegister — multi-value register that retains every concurrent assignment instead of arbitrating. Reader sees the set of "currently competing" values and decides resolution at read-time (e.g. show user a conflict UI).
• GCounterMap — per-key grow-only counter. Pattern: per-tag event counts, per-route hit counter.

Components:

File	Task
`src/crdt/{LWWMap,ORMap,MVRegister,GCounterMap}.ts` (new)	Each implements `Crdt` with toJSON / fromJSON.
`src/crdt/index.ts`	Re-export the four new types + JSON shapes.
`src/crdt/DistributedData.ts`	`decodeCrdt` switch needs four more cases.
`tests/unit/crdt/CrdtProperties.test.ts`	Property tests for each new type (idempotent / commutative / associative).
`tests/unit/crdt/Maps.test.ts` (new)	Hand-rolled scenarios — concurrent put on different keys, concurrent put on same key, ORMap with nested ORSet.

Estimate: 3-4 days.

Verification:

• Each CRDT passes the same three property tests as the existing five.
• Multi-node convergence tests in `tests/multi-node/distributed-data.test.ts` extended to cover at least `LWWMap` + `ORMap`.

Out of scope:

• RGA / Treedoc (collaborative text) — niche, separate issue if demand surfaces.
• Delta-CRDTs — deferred globally.