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.

ArtilleryShell.gd

@@ -0,0 +1,112 @@
+class_name ArtilleryShell
+extends Node2D
+
+# Self-contained telegraph -> travel -> explode sequence for the artillery
+# enemy's attack (see EnemyArtilleryComponent). Lives independently of the
+# enemy that fired it once launched - added to the tree root, exactly like
+# EnemyProjectile/Projectile - so the shooter dying mid-flight doesn't
+# cancel a shot already telegraphed to the player.
+#
+# Shield interaction: a shield charge absorbs this hit entirely, same
+# all-or-nothing semantics as every other damage source in this game
+# (VesselShieldComponent.absorb_hit) - _explode() just calls
+# Ship.take_damage() like everything else and lets that logic decide.
+
+signal finished
+
+@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 color: Color = Color(1.0, 0.5, 0.2)
+
+enum Phase { TELEGRAPHING, TRAVELING, EXPLODING }
+
+const EXPLOSION_DURATION: float = 0.8
+
+var phase: int = Phase.TELEGRAPHING
+var target: Vector2 = Vector2.ZERO
+var shell_pos: Vector2 = Vector2.ZERO
+var telegraph_left: float = 0.0
+var explosion_time: float = 0.0
+var trail: PackedVector2Array = PackedVector2Array()
+
+
+func _ready() -> void:
+	add_to_group("artillery_shells")
+
+
+func launch(from: Vector2, at: Vector2) -> void:
+	target = at
+	shell_pos = from
+	telegraph_left = telegraph_time
+	phase = Phase.TELEGRAPHING
+	position = target
+	queue_redraw()
+
+
+func _process(delta: float) -> void:
+	match phase:
+		Phase.TELEGRAPHING:
+			telegraph_left -= delta
+			if telegraph_left <= 0.0:
+				phase = Phase.TRAVELING
+				trail = PackedVector2Array([shell_pos])
+			queue_redraw()
+		Phase.TRAVELING:
+			var to_target: Vector2 = target - shell_pos
+			var step: float = shell_speed * delta
+			if to_target.length() <= step:
+				shell_pos = target
+				_explode()
+			else:
+				shell_pos += to_target.normalized() * step
+				trail.append(shell_pos)
+				while trail.size() > 14:
+					trail.remove_at(0)
+			position = shell_pos
+			queue_redraw()
+		Phase.EXPLODING:
+			explosion_time -= delta
+			queue_redraw()
+			if explosion_time <= 0.0:
+				finished.emit()
+				queue_free()
+
+
+func _explode() -> void:
+	phase = Phase.EXPLODING
+	explosion_time = EXPLOSION_DURATION
+	position = target
+	for ship in get_tree().get_nodes_in_group("player_ships"):
+		if is_instance_valid(ship) and ship.global_position.distance_to(target) <= aoe_radius:
+			ship.take_damage(damage)
+
+
+func _draw() -> void:
+	match phase:
+		Phase.TELEGRAPHING:
+			var progress: float = clampf(1.0 - telegraph_left / maxf(telegraph_time, 0.001), 0.0, 1.0)
+			# Growing ring plus a filling countdown arc: the warning reads
+			# clearly without needing a number on screen.
+			var radius: float = lerpf(24.0, aoe_radius, progress)
+			_draw_ring(radius, 0.7)
+			draw_arc(Vector2.ZERO, aoe_radius + 14.0, -PI / 2.0, -PI / 2.0 + TAU * progress, 48,
+				Color(color, 0.85), 8.0, true)
+		Phase.TRAVELING:
+			for i in range(1, trail.size()):
+				var alpha: float = float(i) / trail.size()
+				draw_line(trail[i - 1] - shell_pos, trail[i] - shell_pos, Color(color, alpha * 0.6), 5.0, true)
+			draw_circle(Vector2.ZERO, 14.0, Color(color, 0.35))
+			NeonDraw.draw_glowing_circle(self, Vector2.ZERO, 8.0, color, 1.0)
+		Phase.EXPLODING:
+			var eprogress: float = 1.0 - explosion_time / EXPLOSION_DURATION
+			var eradius: float = lerpf(30.0, aoe_radius, minf(eprogress * 1.5, 1.0))
+			draw_circle(Vector2.ZERO, eradius, Color(color, (1.0 - eprogress) * 0.18))
+			_draw_ring(eradius, 1.0 - eprogress)
+
+
+func _draw_ring(radius: float, alpha: float) -> void:
+	draw_arc(Vector2.ZERO, radius, 0.0, TAU, 64, Color(color, 0.16 * alpha), 16.0, true)
+	draw_arc(Vector2.ZERO, radius, 0.0, TAU, 64, Color(color, 0.5 * alpha), 6.0, true)
+	draw_arc(Vector2.ZERO, radius, 0.0, TAU, 64, Color(1.0, 0.92, 0.86, 0.9 * alpha), 2.0, true)