- Add SiegeBot to engine/bot_strategies.go with spawn denial logic
- Implement standalone siege bot in bots/siege/ with main.go, strategy.go, Dockerfile
- Register siege bot in acb-local for arena testing
- Add test-siege-arena.sh script for validation
Strategy: Surround enemy cores to block spawning phase. Bot assigns
units to lockout rings (8 neighbors) around cores, greedily by distance.
Unassigned units collect energy or rush fully-sieged cores.
Tested: 3/10 wins vs rusher+gatherer, 1/10 wins vs rusher+gatherer+guardian
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