#99 (#97a): buildings have health, and can be destroyed

Nothing can attack a structure that cannot be hurt, so this is the floor the
rest of #97 stands on.

max_hp is per definition - an extractor bolted to a rock is not the
mothership - and the current value lives on the RECORD in GameState, not on
the Building node. Main.tscn is torn down and rebuilt on every trip to a
planet surface, so damage kept on a node would heal itself the moment the
player landed.

A record written before this carries no hp field at all. That reads as
undamaged rather than as zero, or an existing save would open on a field of
wrecks.

The mothership cannot be destroyed - it floors at 1hp. The bank, the hangar
and research all live there and a wiped fleet rebuilds there, so losing it
would be a soft-lock with no route back. Keyed on the "bank" capability
rather than on the type: whatever building holds the bank is the one that
cannot be lost.

Destroying a hub retires its transporters. Everything else keyed on a
building id already copes with the building being gone - a transporter
mid-delivery puts its cargo back rather than destroying it.

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

buildings/BuildingField.gd

@@ -22,9 +22,27 @@ var spawned: Array = []
 
 
 func _ready() -> void:
+	# #99: the record is the truth, so the node follows it out. Connected
+	# here rather than in Building.gd because the field owns `spawned` and
+	# would otherwise be left holding a freed node.
+	if not GameState.building_destroyed.is_connected(_on_building_destroyed):
+		GameState.building_destroyed.connect(_on_building_destroyed)
 	rebuild_all()
 
 
+func _on_building_destroyed(building_id: int, _type: String) -> void:
+	var kept: Array = []
+	for b in spawned:
+		if not is_instance_valid(b):
+			continue
+		if int(b.building_id) == building_id:
+			BuildingExplosionVFX.spawn(get_tree().root, b.global_position)
+			b.queue_free()
+			continue
+		kept.append(b)
+	spawned = kept
+
+
 func rebuild_all() -> void:
 	for b in spawned:
 		if is_instance_valid(b):