Harden LWW register state validation

Description

The pitch.

## Summary

Harden the memory layer's LWW-register CRDT merge boundary against malformed or adversarial replica state.

This rejects:
- invalid base64 payloads
- non-string payload fields
- negative Lamport clocks
- empty node IDs
- non-string node IDs

It also adds regression coverage proving malformed remote CRDT state cannot poison an existing trusted replica value.

**Problem:** `02-memory-crdt-lww`  
**Layer:** `memory`

## Validation

Ran the required CI sequence locally:

- `uv sync`
- `uv run ruff check .`