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.

tests/_artillery_telegraph_rocky_preview.gd

@@ -0,0 +1,20 @@
+extends Node2D
+
+
+func _ready() -> void:
+	GameState.planet_name = "Kepler's Rest"
+	GameState.planet_id = "artillery_preview_rocky"
+	GameState.planet_definition_id = "rocky"
+	GameState.surviving_ship_count = 3
+	var surface: Node = load("res://PlanetSurface.tscn").instantiate()
+	surface.get_node("SurfaceController").fade_duration = 0.0
+	add_child(surface)
+
+	var shell: Node = load("res://ArtilleryShell.tscn").instantiate()
+	shell.telegraph_time = 10.0   # frozen mid-countdown for the screenshot
+	shell.aoe_radius = 220.0
+	surface.add_child(shell)
+	shell.launch(Vector2(-600, -600), Vector2.ZERO)
+	shell.telegraph_left = shell.telegraph_time * 0.45
+	shell.set_process(false)
+	shell.queue_redraw()