#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/Building.gd

@@ -14,6 +14,18 @@ var building_type: String:
 	set(value): definition_id = value
 
 
+# #99: drawn only once the structure has actually been hit. A full bar over
+# every building would be permanent clutter saying nothing, and the fleet
+# would be flying over a field of meters in peacetime.
+const HEALTH_BAR_WIDTH: float = 74.0
+const HEALTH_BAR_HEIGHT: float = 7.0
+const HEALTH_BAR_OFFSET: float = -78.0
+const HEALTH_FULL_COLOR: Color = Color(0.45, 0.9, 0.55)
+const HEALTH_HURT_COLOR: Color = Color(1.0, 0.35, 0.25)
+
+var _drawn_health: float = 1.0
+
+
 func _ready() -> void:
 	add_to_group("buildings")
 	z_index = -1
@@ -24,6 +36,41 @@ func _ready() -> void:
 	get_node("VisualHost").add_child(visual)
 
 
+func health_fraction() -> float:
+	return GameState.building_health_fraction(building_id) if building_id != 0 else 1.0
+
+
+func is_damaged() -> bool:
+	return health_fraction() < 0.999
+
+
+func _process(_delta: float) -> void:
+	# Redraw only when the number moves. A building that is not being shot at
+	# costs nothing, which is the same bargain Ship.gd's constant glow makes.
+	var health: float = health_fraction()
+	if is_equal_approx(health, _drawn_health):
+		return
+	_drawn_health = health
+	queue_redraw()
+
+
+func _draw() -> void:
+	if not is_damaged():
+		return
+	# Above the structure rather than across it: the visual scenes all draw
+	# around their own origin, and a bar laid over one would be read as part
+	# of the building.
+	var health: float = health_fraction()
+	var left: float = -HEALTH_BAR_WIDTH * 0.5
+	var top: float = HEALTH_BAR_OFFSET
+	draw_rect(Rect2(left, top, HEALTH_BAR_WIDTH, HEALTH_BAR_HEIGHT),
+		Color(0.02, 0.02, 0.06, 0.85))
+	draw_rect(Rect2(left, top, HEALTH_BAR_WIDTH * health, HEALTH_BAR_HEIGHT),
+		HEALTH_HURT_COLOR.lerp(HEALTH_FULL_COLOR, health))
+	draw_rect(Rect2(left, top, HEALTH_BAR_WIDTH, HEALTH_BAR_HEIGHT),
+		Color(0.55, 0.78, 0.86, 0.55), false, 1.0)
+
+
 func available_actions() -> PackedStringArray:
 	if visual == null:
 		return PackedStringArray()