feat(engine): fix 2-player combat density with tighter spawns and aggressive zone

Problem: 2-player matches had zero combat_death events. Recent combat density
work (bf-4tfh) addressed 3+ player matches but 2-player duels still had zero
combat.

Root cause: 25% spawn radius on 40x40 grid placed bots ~20 tiles apart, with
default zone timing (start turn 50) giving ample time to avoid contact.

Solution:
- Reduce 2-player spawn radius to 20% (from 25%) → ~16 tiles apart at spawn
- Increase 2-player attack radius to 3.5 tiles (from 3) → AttackRadius2=12
- Aggressive 2-player zone: start turn 30, shrink every 2 turns, min radius 3

Results:
- 2-player: 10/10 seeds have combat_death (was 0/10), avg 2.0 per match
- 3-player: 80% have combat_death (was 70%), no regression
- 4-player: 100% have combat_death, avg 3.6 (was 3.2), no regression

Closes: bf-5w8z

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

engine/match.go

@@ -255,11 +255,18 @@ func (mr *MatchRunner) generateMap(gs *GameState, numPlayers int) {
 	}
 
 	// Place cores for each player using rotational symmetry.
-	// Very tight spawn radius (25% from center) forces immediate bot contact.
-	// At 25% radius on 54x54 grid: ~7 tiles from center, ~12 tiles between 3 players.
-	// Attack radius is 3 tiles, so bots need to close ~9 tiles to engage.
-	primaryRadius := 0.25
-	secondaryRadius := 0.15
+	// Tight spawn radius forces immediate bot contact.
+	// For 2 players: 20% from center (~8 tiles on 40x40) → ~16 tiles apart at spawn
+	// For 3+ players: 25% from center (~7 tiles on 54x54) → ~12 tiles apart at spawn
+	// Attack radius is 3 tiles, so bots need to close ~10 tiles to engage.
+	var primaryRadius, secondaryRadius float64
+	if numPlayers == 2 {
+		primaryRadius = 0.20 // Tighter for 2-player to force contact
+		secondaryRadius = 0.12
+	} else {
+		primaryRadius = 0.25
+		secondaryRadius = 0.15
+	}
 	halfRows := float64(centerRow)
 	halfCols := float64(centerCol)
 

engine/types.go

@@ -235,11 +235,17 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 	// Scale energy nodes with player count
 	cfg.EnergyInterval = 10
 
-	// Scale zone parameters for 3+ players to force combat contact
-	// 2-player matches naturally converge at center; 3+ need aggressive zone
+	// Scale zone parameters to force combat contact
 	// Zone must compress bots into contact range while preserving enough population for combat
-	// Tight spawn radius (0.25) means bots start very close; zone finishes the job
-	if numPlayers >= 3 {
+	// For 2-player: start later to allow buildup, then shrink aggressively
+	// Also increase attack radius for 2-player to make combat more likely
+	if numPlayers == 2 {
+		cfg.ZoneStartTurn = 30     // Allow time for bots to spawn and move
+		cfg.ZoneShrinkInterval = 2 // Shrink every 2 turns (faster than 3+)
+		cfg.ZoneShrinkStep = 2     // Shrink 2 tiles per interval (1 tile/turn)
+		cfg.ZoneMinRadius = 3      // Tight final zone forces contact
+		cfg.AttackRadius2 = 12     // 3.5 tiles (balanced for 2-player)
+	} else {
 		cfg.ZoneStartTurn = 15     // Start zone early (bots already close at spawn)
 		cfg.ZoneShrinkInterval = 3 // Shrink every 3 turns (vs default 5)
 		cfg.ZoneShrinkStep = 3     // Shrink 3 tiles per interval (1 tile/turn)