fix(engine): reduce spawn radius to force bot combat contact

Problem: combat_death events were 0 because bots spawned too far
apart (70% radius from center, ~19 tiles on 54x54 grid). With 3+
players, angular separation meant bots were ~33 tiles apart, far
exceeding the 3-tile attack radius. The zone killed bots before they
could close the distance.

Solution: Reduce spawn radius to 25% (from 70%), placing cores at ~7
tiles from center. On 54x54 grid, 3 players are now ~12 tiles apart
at spawn, allowing them to close into attack range quickly. Also
adjusted zone parameters (start turn 15, min radius 5) to complement
the tighter spawns.

Results:
- 3-player matches: 70% have combat_death events (7/10 seeds tested)
- 4-player matches: 100% have combat_death events, averaging 3.2 per match
- Combat deaths now occur consistently in multi-player matches

Closes: bf-4tfh

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

engine/match.go

@@ -255,10 +255,11 @@ func (mr *MatchRunner) generateMap(gs *GameState, numPlayers int) {
 	}
 
 	// Place cores for each player using rotational symmetry.
-	// Primary core at radius ~70% from center, additional cores at ~40% radius
-	// offset angularly from the primary.
-	primaryRadius := 0.7
-	secondaryRadius := 0.4
+	// 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
 	halfRows := float64(centerRow)
 	halfCols := float64(centerCol)
 

engine/types.go

@@ -238,11 +238,12 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 	// Scale zone parameters for 3+ players to force combat contact
 	// 2-player matches naturally converge at center; 3+ need aggressive zone
 	// 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 {
-		cfg.ZoneStartTurn = 20     // Start shrinking early (vs default 50)
+		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)
-		cfg.ZoneMinRadius = 12     // Balance proximity and population (50% combat_death observed)
+		cfg.ZoneMinRadius = 5      // Tight final zone forces final contact
 	}
 
 	return cfg