#138: structures and doctrines cost more than rocks, and more the deeper they are

Both halves of the ticket, in the shape vessels have used since gas bombers
and laser ships: a definition carries an ore figure and, when it wants more
than ore, a dictionary that IS the price. `building_costs()` and
`skill_costs()` answer it the way `product_costs()` already did, so there is
one question with one answer whatever is being bought, and every payment
path now goes through `spend_resources` rather than `spend`.

Seven of ten structures and thirteen of seventeen doctrines now want gas,
energy or dark matter alongside their rocks, and the totals climb with tree
depth - structures 16/31/58/62/96 by depth, doctrines 15/30/73/96/143/258/530.

The constraint that shaped the table is REACHABILITY, and it is the reason
some prices stayed ore-only. Energy exists only where a gas burner stands,
and a gas burner needs Power Systems - so nothing at or before that depth
may be priced in energy, or it is a wall with no way through. Dark matter is
rarer still and appears once, on the deepest node in the game. The extractor
stays rocks-only despite its depth allowing gas, because the quest chain
reaches it before it has sent the player for any.

test_progressive_costs checks those as properties rather than transcribing
the numbers, so rebalancing a price costs nothing while breaking the shape
fails: nothing unreachable at its own depth, prices rising with depth, the
ore field agreeing with the dictionary (repairs quote the field alone, since
a repair drone carries ore and nothing else), and the quest chain never
asking for a structure it has not funded.

Two things cleaned up on the way:

- The per-building cost constants in GameState are gone. Nothing but tests
  used them, and they were a second copy of a number the definitions already
  hold - exactly what would drift once a price gained a resource.
- research_for_building() returns a display NAME, and the id-keyed half of
  the tree wants an id. That mix-up is silent - skill_depth() of a name finds
  no node and answers 0, so a deep structure reads as an ungated one, which
  is how the first draft of the test let an unreachable price through.
  research_node_for_building() is the id, and the name is expressed from it.

PriceTag no longer rebuilds its labels when re-told the same price; it
refreshes the held figures instead. The craft menu re-states prices every
frame and the build menu now does too, so that was a dozen Labels being torn
down and rebuilt sixty times a second.

tests/test_mothership_replacement.gd

@@ -111,7 +111,7 @@ func _process(delta: float) -> bool:
 			controller._confirm_build()
 			var only_one: bool = _count_motherships() == 1
 			var moved: bool = game_state.get_mothership_position().distance_to(second_pos) < 60.0
-			var charged: bool = game_state.rocks == rocks_before - GameState.MOTHERSHIP_COST
+			var charged: bool = game_state.rocks == rocks_before - int(game_state.building_costs(GameState.BUILDING_MOTHERSHIP)[GameState.RESOURCE_ORE])
 			print("confirmed: motherships=%d moved=%s rocks %d->%d charged=%s" % [
 				_count_motherships(), moved, rocks_before, game_state.rocks, charged
 			])
@@ -121,7 +121,7 @@ func _process(delta: float) -> bool:
 				return true
 
 			# --- unaffordable replacement must not destroy the old one ---
-			game_state.rocks = GameState.MOTHERSHIP_COST - 1
+			game_state.rocks = int(game_state.building_costs(GameState.BUILDING_MOTHERSHIP)[GameState.RESOURCE_ORE]) - 1
 			_place_fleet(Vector2(2500.0, -800.0))
 			controller._on_build_pressed()
 			controller._show_building_info(GameState.BUILDING_MOTHERSHIP)