#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>

vfx/BuildingExplosionVFX.gd

@@ -0,0 +1,22 @@
+class_name BuildingExplosionVFX
+extends LightBurstVFX
+
+
+func _init() -> void:
+	# #99: bigger and slower than a hull going up. A structure coming down is
+	# the largest thing the player can lose without losing the run, and it
+	# has to be legible from across the map rather than mistaken for another
+	# kill in the fight that caused it.
+	color = Color(1.0, 0.6, 0.2)
+	particle_count = 34
+	lifetime = 1.05
+	min_speed = 90.0
+	max_speed = 520.0
+	shockwave_radius = 190.0
+	shockwave_color = Color(1.0, 0.78, 0.35)
+
+
+static func spawn(parent: Node, world_position: Vector2) -> void:
+	var vfx: BuildingExplosionVFX = BuildingExplosionVFX.new()
+	vfx.position = world_position
+	parent.add_child(vfx)