- Initialize lastDamagedTick to -1e9 for fresh units (finite sentinel to avoid JSON.stringify null)
- Add lastDamagedTick to serialize/deserialize so the pause timer persists across save/load
- Add comprehensive tests for self-heal behavior:
- Rate verification (~33% max HP/min)
- 30s pause after damage and clock restart on new hits
- Sustained fire suppression
- No overhealing
- Non-selfHeal units don't regenerate
- Damage clock survives save/load round-trip
