feat(engine): reduce default map size to 40x40 and add skirmish map class

Reduce default 2-player map size from 60x60 to 40x40 (from 3600 to 1600
tiles) to increase encounter frequency and combat density. Add -skirmish
flag to acb-mapgen for generating even smaller dense maps (32x32, 0.20
wall density, 15 energy nodes) with "skirmish_" ID prefix.

Changes:
- engine/types.go: DefaultConfig() returns 40x40, ConfigForPlayers()
  uses 800 tiles/player for 2-player (40x40) and 2000 tiles/player for
  3+ players
- cmd/acb-matchmaker/tickers.go: gridForPlayers() returns 40x40 for 2
  players
- cmd/acb-map-evolver/main.go: gridForPlayers() returns 40x40 for 2
  players
- cmd/acb-mapgen/main.go: defaults to 40x40, adds -skirmish flag for
  32x32 high-density maps
- cmd/acb-matchmaker/tickers_test.go: update test expectations for new
  40x40 default

Closes: bf-39wt

cmd/acb-map-evolver/main.go

@@ -1047,9 +1047,11 @@ func (e *MapEvolver) saveMap(ctx context.Context, m *Map) error {
 }
 
 // gridForPlayers returns default grid dimensions for a given player count.
+// For 2-player matches, uses 40x40 (down from 60x60) to increase encounter frequency.
+// For 3+ players, targets ~2000 tiles per player.
 func gridForPlayers(n int) (rows, cols int) {
 	if n <= 2 {
-		return 60, 60
+		return 40, 40
 	}
 	side := int(math.Sqrt(float64(2000 * n)))
 	if side < 40 {

cmd/acb-mapgen/main.go

@@ -39,13 +39,14 @@ type Core struct {
 func main() {
 	// Command-line flags
 	players := flag.Int("players", 2, "Number of players (2, 3, 4, or 6)")
-	rows := flag.Int("rows", 60, "Grid rows")
-	cols := flag.Int("cols", 60, "Grid columns")
+	rows := flag.Int("rows", 40, "Grid rows")
+	cols := flag.Int("cols", 40, "Grid columns")
 	wallDensity := flag.Float64("wall-density", 0.15, "Wall density (0.0-0.3)")
 	energyNodes := flag.Int("energy-nodes", 20, "Energy nodes")
 	seed := flag.Int64("seed", time.Now().UnixNano(), "Random seed")
 	output := flag.String("output", "", "Output file (default: stdout)")
 	maxAttempts := flag.Int("max-attempts", 100, "Max attempts to generate a connected map")
+	skirmish := flag.Bool("skirmish", false, "Generate a skirmish map (32x32, higher density)")
 	help := flag.Bool("help", false, "Show help")
 
 	flag.Usage = func() {
@@ -58,6 +59,8 @@ func main() {
 		fmt.Fprintf(flag.CommandLine.Output(), "  3 players: 120° rotational\n")
 		fmt.Fprintf(flag.CommandLine.Output(), "  4 players: 90° rotational\n")
 		fmt.Fprintf(flag.CommandLine.Output(), "  6 players: 60° rotational\n\n")
+		fmt.Fprintf(flag.CommandLine.Output(), "Map presets:\n")
+		fmt.Fprintf(flag.CommandLine.Output(), "  -skirmish    Small dense map (32x32, 0.20 wall density, 15 energy)\n\n")
 		fmt.Fprintf(flag.CommandLine.Output(), "Options:\n")
 		flag.PrintDefaults()
 	}
@@ -69,6 +72,14 @@ func main() {
 		os.Exit(0)
 	}
 
+	// Apply skirmish preset if requested
+	if *skirmish {
+		*rows = 32
+		*cols = 32
+		*wallDensity = 0.20
+		*energyNodes = 15
+	}
+
 	// Validate player count
 	validPlayers := map[int]bool{2: true, 3: true, 4: true, 6: true}
 	if !validPlayers[*players] {
@@ -91,8 +102,12 @@ func main() {
 		os.Exit(1)
 	}
 
-	// Generate map ID
-	m.ID = generateMapID(rng)
+	// Generate map ID with skirmish prefix if applicable
+	if *skirmish {
+		m.ID = generateMapIDWithPrefix(rng, "skirmish")
+	} else {
+		m.ID = generateMapID(rng)
+	}
 	m.Generated = time.Now().UTC()
 
 	// Output
@@ -114,12 +129,16 @@ func main() {
 }
 
 func generateMapID(rng *rand.Rand) string {
+	return generateMapIDWithPrefix(rng, "map")
+}
+
+func generateMapIDWithPrefix(rng *rand.Rand, prefix string) string {
 	const chars = "abcdefghijklmnopqrstuvwxyz0123456789"
 	b := make([]byte, 8)
 	for i := range b {
 		b[i] = chars[rng.Intn(len(chars))]
 	}
-	return "map_" + string(b)
+	return prefix + "_" + string(b)
 }
 
 func generateMap(numPlayers, rows, cols int, wallDensity float64, numEnergyNodes int, rng *rand.Rand) *Map {

cmd/acb-matchmaker/tickers.go

@@ -348,11 +348,12 @@ func (m *Matchmaker) selectMapLRU(ctx context.Context, playerCount int, rng *ran
 	return fmt.Sprintf("map_%d", seed%100000), rows, cols, seed
 }
 
-// gridForPlayers returns default grid dimensions for a given player count,
-// mirroring the formula in engine.ConfigForPlayers (~2000 tiles per player).
+// gridForPlayers returns default grid dimensions for a given player count.
+// For 2-player matches, uses 40x40 (down from 60x60) to increase encounter frequency.
+// For 3+ players, targets ~2000 tiles per player.
 func gridForPlayers(n int) (rows, cols int) {
 	if n <= 2 {
-		return 60, 60
+		return 40, 40
 	}
 	side := int(math.Sqrt(float64(2000 * n)))
 	if side < 40 {

cmd/acb-matchmaker/tickers_test.go

@@ -187,7 +187,7 @@ func TestGridForPlayers(t *testing.T) {
 		minArea int
 		maxArea int
 	}{
-		{2, 3000, 4200},   // 60x60 = 3600
+		{2, 1200, 2000},   // 40x40 = 1600 (reduced from 60x60 for combat density)
 		{3, 4000, 6000},
 		{4, 5000, 8500},
 		{6, 7000, 12000},

engine/types.go

@@ -182,8 +182,8 @@ type Config struct {
 // DefaultConfig returns the default game configuration.
 func DefaultConfig() Config {
 	return Config{
-		Rows:             60,
-		Cols:             60,
+		Rows:             40,
+		Cols:             40,
 		MaxTurns:         500,
 		VisionRadius2:    49, // ~7 tiles
 		AttackRadius2:    5,  // ~2.24 tiles
@@ -199,7 +199,8 @@ func DefaultConfig() Config {
 }
 
 // ConfigForPlayers returns a config scaled for the given player count and cores per player.
-// Uses ~1800-2000 tiles per player (following aichallenge Ants sizing).
+// For 2 players, uses 40x40 (800 tiles per player) to increase encounter frequency.
+// For 3+ players, uses ~2000 tiles per player (following aichallenge Ants sizing).
 func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 	cfg := DefaultConfig()
 	cfg.CoresPerPlayer = coresPerPlayer
@@ -207,14 +208,19 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 		cfg.CoresPerPlayer = 1
 	}
 
-	// Scale grid: ~2000 tiles per player, square grid
-	areaPerPlayer := 2000
+	// Scale grid: smaller maps for 2-player, ~2000 tiles/player for 3+ players
+	var areaPerPlayer int
+	if numPlayers == 2 {
+		areaPerPlayer = 800 // 40x40 for 2 players
+	} else {
+		areaPerPlayer = 2000
+	}
 	totalArea := areaPerPlayer * numPlayers
 	side := int(math.Sqrt(float64(totalArea)))
 
 	// Clamp to valid range
-	if side < 40 {
-		side = 40
+	if side < 30 {
+		side = 30
 	}
 	if side > 200 {
 		side = 200