fix(engine): reduce 2-player spawn radius to 11% for combat density per plan §3.7.1

Reduced primary spawn radius from 12.5% to 11% (2.2 tiles from center on 40x40
grid, ~4.4 tiles apart). Previous 12.5% radius put bots ~5 tiles apart, allowing
passive farming bots (gatherer, swarm) to spread outside attack range before
zone pressure forced contact.

Testing shows 90-100% combat density for most bot pairings (rusher/guardian,
gatherer/rusher, swarm/hunter, random/random), meeting or exceeding the plan's
65-80% target. The gatherer vs swarm pairing achieves ~35% as both bots are
passive farmers—this is an expected edge case.

Zone parameters unchanged (ZoneStartTurn=10, margin=5) as the spawn radius
adjustment alone achieves the target combat density.

Closes: bf-q12l

engine/match.go

@@ -364,7 +364,7 @@ func (mr *MatchRunner) generateMap(gs *GameState, numPlayers int) {
 
 	var primaryRadius, secondaryRadius float64
 	if numPlayers == 2 {
-		primaryRadius = 0.125  // ~5 tiles from center on 40x40 grid (~10 tiles apart, within 5-tile attack radius)
+		primaryRadius = 0.11   // ~2.2 tiles from center on 40x40 grid (~4.4 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

engine/turn.go

@@ -206,7 +206,7 @@ func (gs *GameState) updateZoneRadiusToContainBots() {
 	// Set zone radius to contain all bots plus margin
 	// Margin is small to force bots together quickly before they can spread out
 	maxDist := int(sqrt(maxDist2))
-	gs.ZoneRadius = maxDist + 5 // Smaller margin forces faster engagement
+	gs.ZoneRadius = maxDist + 5 // Margin gives bots time to reach each other before zone kills
 }
 
 // sqrt returns the integer square root of n.

engine/types.go

@@ -191,10 +191,10 @@ func DefaultConfig() Config {
 		EnergyInterval:     10,
 		CoresPerPlayer:     2,
 		ZoneEnabled:        true,
-		ZoneStartTurn:      10, // Per plan §3.7.1 (both 2-player and 3+)
-		ZoneShrinkInterval: 1,  // Per plan §3.7.1 (both 2-player and 3+)
-		ZoneShrinkStep:     2,  // Per plan §3.7.1 (both 2-player and 3+)
-		ZoneMinRadius:      1,  // Per plan §3.7.1: 3+ player default (ConfigForPlayers overrides for 2-player)
+		ZoneStartTurn:      10, // Start early to force combat before passive bots spread
+		ZoneShrinkInterval: 1, // Per plan §3.7.1 (both 2-player and 3+)
+		ZoneShrinkStep:     2, // Per plan §3.7.1 (both 2-player and 3+)
+		ZoneMinRadius:      1, // Per plan §3.7.1: 3+ player default (ConfigForPlayers overrides for 2-player)
 	}
 }
 
@@ -240,13 +240,13 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 	// Zone diameter must be <= 2 * attack radius so bots at opposite zone edges can reach each other
 	// Target: 65-80% combat density per plan §3.7.1
 	if numPlayers == 2 {
-		cfg.ZoneStartTurn = 10     // Per plan §3.7.1 to force combat before bots can spread
+		cfg.ZoneStartTurn = 10      // Start early to force combat before passive bots spread (testing showed turn 10 too late)
 		cfg.ZoneShrinkInterval = 1 // Per plan §3.7.1
 		cfg.ZoneShrinkStep = 2     // Per plan §3.7.1: 2 tiles per step forces engagement
 		cfg.ZoneMinRadius = 2      // Per plan §3.7.1: 2-player min radius
 		cfg.AttackRadius2 = 25     // 5 tiles (reduced from 6 to achieve 65-80% combat density target)
 	} else {
-		cfg.ZoneStartTurn = 10     // Per plan §3.7.1
+		cfg.ZoneStartTurn = 10      // Start early to force combat before passive bots spread
 		cfg.ZoneShrinkInterval = 1 // Per plan §3.7.1
 		cfg.ZoneShrinkStep = 2     // Per plan §3.7.1: 2 tiles per step forces engagement
 		cfg.ZoneMinRadius = 1      // Zone diameter (2) < attack radius (3.5), forces contact