feat: Add spawn position caching and spiral scan optimization (#2295)

### Summary

Adds spawn position caching and optimization for constrained spawners (e.g., those near houses, water, or blocked terrain).

### Key features:
- Sector-based bitmap cache (32 bytes per 16x16 sector) stores valid spawn positions
- Spiral scan progressively discovers positions from spawner center outward
- Automatic mode detects constrained spawners after 5+ non-transient failures
- Prevents mob spawning inside private houses (allows public AoS buildings)
- Deduplicates sector lookups for multi-bounds spawners (RegionSpawner)
- Cache invalidation on house placement/demolition
- Moves SpawnBounds to Spawner

### New spawner properties:
- SpawnPositionMode: Automatic (default), Enabled, Disabled, Abandoned
- MaxSpawnAttempts: Configurable attempts before optimization engages (default: 5)

Projects/UOContent/Network/EntityPackets.cs

@@ -13,7 +13,6 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
  *************************************************************************/
 
-using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;