The zone was killing bots at spawn radius before they could close distance
and engage in combat. With the old parameters (zone start turn 30, min radius 3),
bots were eliminated by the zone before reaching attack range.
Changes:
- 2-player: zone start 30→60, shrink interval 2→3, min radius 3→5
- 3+ player: zone start 30→50, shrink step 3→2, min radius 3→8
- ZoneMinRadius now >= spawn radius so bots survive to final zone
Verification:
- Test replay 1 (seed 12345): combat_death events at turn 5
- Test replay 2 (seed 42): 44 combat_death events across 36 turns
This fixes the combat-density issue where matches played out as pure
energy-farming with zero combat_death events.
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