#101 (#97c): waves attack buildings

Every enemy component used to ask the same question - which player SHIP is
nearest - so a structure was scenery they flew straight through. A Building
is also a plain Node2D, so there was nothing to collide with even if one had
tried.

EnemyTargets answers the question once, for pursuit, standoff, the lancer's
lunge and the ranged weapon together, so they cannot drift into disagreeing
about it. It keeps one thumb on the scale: a vessel inside 520px always
wins, even against a nearer building, or the fleet could not defend anything
by interposing itself and an enemy would fly past the ships shooting it.

Buildings get a hitbox - an Area2D in a group, the same shape as a Ship or
an asteroid, so EnemyProjectile needed one extra branch rather than a new
concept. It applies damage on a tick rather than on area_entered: entry
fires once, which is right for a rammer that dies on the first strike, but
would let a sniper that drifted inside sit there scratch-free for ever. On a
tick, an enemy parked on a structure is besieging it.

What one strike costs is data on the enemy definition, for the same reason
dies_on_contact is - a lancer arriving at 950px/s is not a sniper drifting
into a wall.

Also clears the asteroids in test_enemy_projectile_damage. set_process(false)
only stops NEW asteroids; the ones already placed keep drifting, and one
wandering into the shield vessel parked at (2000, 0) spent its single charge
so the projectile fired next killed it. That failed about one parallel suite
run in thirty and never in isolation. Clearing them is what the other combat
tests already do.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

enemies/EnemyTargets.gd

@@ -0,0 +1,65 @@
+class_name EnemyTargets
+extends RefCounted
+
+# #101 (#97c): who a wave enemy is coming for.
+#
+# Before this, every enemy component asked the same question - "which player
+# SHIP is nearest?" - and a structure was scenery they flew straight past.
+# #97 makes waves go after what the player has built, so the question
+# becomes "which player thing is nearest", asked in one place so pursuit,
+# standoff, the lancer's lunge and the ranged weapon cannot drift into
+# disagreeing about it.
+#
+# There is one thumb on the scale. A ship inside SHIP_PRIORITY_RANGE always
+# wins, even if a building is nearer: an enemy that ignored the vessels
+# shooting it in order to reach a depot two hundred pixels closer would read
+# as broken, and would make the fleet unable to defend anything by
+# interposing itself.
+
+const SHIP_PRIORITY_RANGE: float = 520.0
+
+
+static func nearest_ship(node: Node, from: Vector2) -> Node2D:
+	var fleet: Node2D = node.get_tree().get_first_node_in_group("player_fleet")
+	if fleet == null:
+		return null
+	var nearest: Node2D = null
+	var best: float = INF
+	for ship in fleet.ships:
+		if not is_instance_valid(ship):
+			continue
+		var d: float = from.distance_to(ship.global_position)
+		if d < best:
+			best = d
+			nearest = ship
+	return nearest
+
+
+static func nearest_building(node: Node, from: Vector2) -> Node2D:
+	var nearest: Node2D = null
+	var best: float = INF
+	for building in node.get_tree().get_nodes_in_group("buildings"):
+		if not is_instance_valid(building) or building.is_queued_for_deletion():
+			continue
+		var d: float = from.distance_to(building.global_position)
+		if d < best:
+			best = d
+			nearest = building
+	return nearest
+
+
+static func nearest_target(node: Node, from: Vector2) -> Node2D:
+	var ship: Node2D = nearest_ship(node, from)
+	if ship != null and from.distance_to(ship.global_position) <= SHIP_PRIORITY_RANGE:
+		return ship
+	var building: Node2D = nearest_building(node, from)
+	if building == null:
+		return ship
+	if ship == null:
+		return building
+	return ship if from.distance_to(ship.global_position) \
+		<= from.distance_to(building.global_position) else building
+
+
+static func is_building(target: Node) -> bool:
+	return target != null and target.is_in_group("buildings")