fix(rust_brain): preserve HLC on snapshot restore and gossip receive

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Bug and impact

restore_from_file() and gossip.receive() did not preserve Hybrid Logical Clock (HLC) timestamps introduced in v0.6.0. After a snapshot restore:

• Restored nodes received fresh HLC timestamps instead of their stored values
• Causal successor writes with pre-crash HLC were rejected with TimestampRegression → silent data loss
• Gossip events without HLC could overwrite fresher data on existing keys

Concrete trigger: Snapshot a node with hlc=(5000, 10, "nodeA"), restore, then attempt remember("k", "v2", hlc=(5000, 11, "nodeA")) — the write fails because restore assigned a fresh wall-clock HLC.

Root cause

restore_from_file() reconstructed MemoryNode without reading the hlc field from snapshot JSON (defaults to _hlc.now()). gossip.receive() similarly omitted HLC passthrough and stale-update rejection.

Fix

• Add _parse_hlc() for snapshot/wire normalisation with legacy ts_ns fallback
• restore_from_file: restore hlc per node and advance global _hlc
• bulk_write: pass through hlc from row payloads
• gossip.receive: apply hlc, reject stale updates, skip missing-HLC overwrites on existing keys

Validation

• Reproduced pre-fix: HLC not preserved, successor write raised TimestampRegression
• Post-fix: HLC round-trip preserved, successor write succeeds
• pytest tests/test_enterprise_backup.py tests/test_gossip.py tests/test_rust_brain.py — 28 passed