feat(ai): ✨ Implement military threat detection and defensive unit purchasing heuristics

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

src/game/engine/src/modules/ai/simple_heuristic_ai.gd

@@ -46,6 +46,10 @@ static func process_player(player: RefCounted) -> Array:
 		player
 	)
 
+	var threat: Dictionary = _enemy_military_threat(player)
+	if bool(threat.get("threatens_city", false)):
+		_rush_buy_defenders(player, threat)
+
 	# Units: founders first (expansion), then military.
 	for idx: int in player.units.size():
 		var unit: Variant = player.units[idx]
@@ -194,6 +198,54 @@ static func _tile_has_enemy_unit(
 	return false
 
 
+static func _enemy_military_threat(player: RefCounted) -> Dictionary:
+	## count=in-range(<=8), total_count=all enemy combat. threatens_city @<=5.
+	var count: int = 0
+	var total: int = 0
+	var nearest: int = INF_DISTANCE
+	var spawn: RefCounted = null
+	for eu: Variant in _collect_enemy_units(player):
+		if int(eu.get("attack")) <= 0 and int(eu.get("ranged_attack")) <= 0:
+			continue
+		total += 1
+		var min_d: int = INF_DISTANCE
+		var best_c: RefCounted = null
+		for c: RefCounted in player.cities:
+			var d: int = HexUtilsScript.hex_distance(eu.position, c.position)
+			if d < min_d: min_d = d; best_c = c
+		if min_d <= 8: count += 1
+		if min_d < nearest: nearest = min_d; spawn = best_c
+	return {"count": count, "total_count": total, "nearest_dist": nearest,
+		"threatens_city": nearest <= 5, "spawn_city": spawn}
+
+
+static func _rush_buy_defenders(player: RefCounted, threat: Dictionary) -> void:
+	## Spawn warriors at threat.spawn_city while gold >= 50 and under
+	## max(2, threat+1) defenders (hard cap 3x cities). Direct-spawn pattern
+	## from auto_play.gd so the bridge needs no new action type.
+	var spawn_city: RefCounted = threat.get("spawn_city")
+	if spawn_city == null: return
+	var mil: int = 0
+	for u: Variant in player.units:
+		if u == null or not u.is_alive() or u.get("can_found_city") == true:
+			continue
+		if int(u.get("attack")) > 0 or int(u.get("ranged_attack")) > 0:
+			mil += 1
+	var target: int = maxi(2, int(threat.get("count", 0)) + 1)
+	var cap: int = player.cities.size() * 3
+	while player.gold >= 50 and mil < target and mil < cap:
+		var nu: RefCounted = UnitScript.new(
+			"warrior", player.index, spawn_city.position
+		)
+		nu.id = "rush_%d_%d" % [GameState.turn_number, mil]
+		nu.display_name = "Warrior"
+		player.units.append(nu)
+		GameState.get_primary_layer().get("units", []).append(nu)
+		player.gold -= 50
+		mil += 1
+		EventBus.unit_created.emit(nu, player.index)
+
+
 # ── Founder logic ────────────────────────────────────────────────────────
 
 
@@ -374,6 +426,19 @@ static func _decide_production(
 	var city: RefCounted = player.cities[city_index]
 	var city_count: int = player.cities.size()
 
+	var threat: Dictionary = _enemy_military_threat(player)
+	var threatened: bool = bool(threat.get("threatens_city", false))
+	var enemy_total: int = int(threat.get("total_count", 0))
+
+	# Threat preemption: when an enemy stack is closing on a city, force
+	# military production over walls/happiness/founders until we can field
+	# at least enemy_total + 1 defenders (matches opponent's full army, not
+	# just the in-range slice — in-range saturates while reserves escalate).
+	if threatened and military_count < maxi(3, enemy_total + 1):
+		var rush_unit: String = _pick_buildable_military_unit_id(city, player)
+		if not rush_unit.is_empty():
+			return _prod_unit(city_index, rush_unit)
+
 	# Priority 0: Emergency garrison — no military at all means the next
 	# enemy stack wins uncontested before any wall is finished. A single
 	# warrior buys ~10 turns of breathing room at a fraction of walls'
@@ -408,8 +473,14 @@ static func _decide_production(
 	):
 		return _prod_unit(city_index, "founder")
 
-	# Priority 4: Military — maintain 2 warriors per city
-	var want_military: bool = military_count < maxi(2, city_count * 2)
+	# Priority 4: Military — maintain 2 warriors per city, scaling up to
+	# match enemy's FULL army when they're closing on us so we don't lose
+	# on parity once reserves arrive.
+	var enemy_mil: int = enemy_total if threatened else 0
+	var mil_target: int = maxi(2, city_count * 2)
+	if enemy_mil > 0:
+		mil_target = maxi(mil_target, enemy_mil + 1)
+	var want_military: bool = military_count < mil_target
 	if want_military:
 		var unit_id: String = _pick_buildable_military_unit_id(city, player)
 		if not unit_id.is_empty():