Add artillery enemy with telegraphed AOE (#46)

EnemyArtilleryComponent is a stationary siege piece (slow drift toward
the fleet, deliberately no pursuit component) that fires at the fleet
centroid on a long cooldown via a standalone ArtilleryShell -
telegraph ring (~1.8s, growing radius plus a filling countdown arc) ->
slow visible travel -> AOE explosion via Ship.take_damage() on every
player_ships node inside, same all-or-nothing shield absorption as
every other damage source. The shell is independent of the enemy that
fired it (added to the tree root, like EnemyProjectile/Projectile) so
the shooter dying mid-flight doesn't cancel an already-telegraphed
shot, and the impact point is fixed at fire time - never re-aimed at
where the fleet moves to afterward.

Part of the #40 conquest-mechanic breakdown.

EnemyArtilleryComponent.gd

@@ -0,0 +1,61 @@
+class_name EnemyArtilleryComponent
+extends Node
+
+# Stationary siege piece: drifts slowly toward the fleet (deliberately no
+# pursuit component - closing distance is incidental, not its purpose) and
+# fires a slow, heavily telegraphed AOE shell on a long cooldown via a
+# standalone ArtilleryShell, so an already-launched shot survives this
+# enemy dying mid-flight, same as EnemyProjectile/Projectile already do.
+
+var capability_id: String = "artillery"
+
+@export var fire_cooldown: float = 6.0
+@export var fire_range: float = 900.0
+@export var telegraph_time: float = 1.8
+@export var shell_speed: float = 180.0
+@export var aoe_radius: float = 220.0
+@export var damage: int = 3
+@export var drift_speed: float = 20.0
+@export var color: Color = Color(1.0, 0.5, 0.2)
+
+var cooldown_left: float = 0.0
+var shell_scene: PackedScene = preload("res://ArtilleryShell.tscn")
+
+
+func tick(delta: float) -> void:
+	var enemy: Node2D = get_parent()
+	_drift(enemy, delta)
+
+	cooldown_left -= delta
+	if cooldown_left > 0.0:
+		return
+
+	var fleet: Node2D = get_tree().get_first_node_in_group("player_fleet")
+	if fleet == null or fleet.ships.is_empty():
+		return
+	var centroid: Vector2 = fleet.get_centroid()
+	if enemy.global_position.distance_to(centroid) > fire_range:
+		return
+
+	var shell: Node2D = shell_scene.instantiate()
+	shell.telegraph_time = telegraph_time
+	shell.shell_speed = shell_speed
+	shell.aoe_radius = aoe_radius
+	shell.damage = damage
+	shell.color = color
+	enemy.get_tree().root.add_child(shell)
+	shell.launch(enemy.global_position, centroid)
+	cooldown_left = fire_cooldown
+
+
+func _drift(enemy: Node2D, delta: float) -> void:
+	# Local position throughout, same convention as EnemyPursuitComponent/
+	# EnemyStandoffComponent: valid as long as the enemy's container and
+	# Fleet share a common origin, which every scene in this project does.
+	var fleet: Node2D = get_tree().get_first_node_in_group("player_fleet")
+	if fleet == null or fleet.ships.is_empty():
+		return
+	var to_fleet: Vector2 = fleet.get_centroid() - enemy.position
+	if to_fleet.length() < 1.0:
+		return
+	enemy.position += to_fleet.normalized() * drift_speed * delta