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

[cursor]

Bug and impact

restore_from_file() and gossip.receive() did not preserve or apply Hybrid Logical Clock (HLC) timestamps introduced in v0.6.0. After disaster-recovery snapshot restore, nodes received fresh wall-clock HLCs instead of their stored causal timestamps. Legitimate post-restore writes and gossip updates with correct successor HLCs were rejected as TimestampRegression, causing silent data loss in backup/restore and multi-node gossip scenarios.

Concrete trigger: Snapshot a node with hlc=(5000, 10, "nodeA"), restore, then apply a causal successor hlc=(5000, 11, "nodeA") — rejected because restore assigned a fresh wall-clock HLC.

Root cause

• restore_from_file() rebuilt MemoryNode without reading the hlc field from the snapshot and never advanced the global _hlc.
• gossip.receive() omitted hlc when calling remember().
• bulk_write() also dropped HLC from row payloads.

Fix

• Add _parse_hlc() to normalise HLC from snapshot/wire form (with legacy ts_ns fallback).
• Restore stored HLC on snapshot load and call _hlc.update() for each restored node.
• Pass HLC through bulk_write and gossip.receive; reject stale gossip overwrites; skip HLC-less gossip updates on existing keys.
• Hold lock during restore_from_file mutation and gc_expired.

Validation

• Reproduced pre-fix failure scenario.
• Added regression tests in test_enterprise_backup.py and test_gossip.py.
• Full suite: 192 passed, 8 skipped.