fix(engine): increase 2-player spawn radius to 25% to prevent turn-1 mutual destruction

Per plan §3.7.1, the target is 65-80% combat density with ~1 death per 20 turns.
Previous 11% spawn radius put bots ~4.4 tiles apart on 40x40 grid, within the
5-tile attack radius (attack_radius2=25), causing immediate mutual destruction
on turn 1. New 25% radius puts bots ~10 tiles apart, outside attack range.

Testing:
- Local match: 4 turns vs 1 turn previously (combat_death events on turn 4)
- Combat density test: 86% matches with combat_deaths (target 65-80%)
- Deaths per turn: 0.355 (still higher than 0.05 target, but zone may contribute)

Closes: bf-53b4

engine/match.go

@@ -364,7 +364,7 @@ func (mr *MatchRunner) generateMap(gs *GameState, numPlayers int) {
 
 	var primaryRadius, secondaryRadius float64
 	if numPlayers == 2 {
-		primaryRadius = 0.11   // ~2.2 tiles from center on 40x40 grid (~4.4 tiles apart)
+		primaryRadius = 0.25   // ~5 tiles from center on 40x40 grid (~10 tiles apart)
 		secondaryRadius = 0.05 // ~2 tiles from center (closer to center for additional cores)
 	} else {
 		primaryRadius = 0.10 // ~5 tiles from center on 50x50 grid