#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_hub_capacity.gd

@@ -12,6 +12,8 @@ func _initialize() -> void:
 	state.transporters.clear()
 	state.reset_progression()
 	state.rocks = 900
+	# #138: doctrines past the first tier want gas as well as rocks.
+	state.bank_inventory.set_amount(GameState.RESOURCE_GAS, 900)
 	state.research_points = state.skill_research_point_cost("freight_yards")
 	state.unlocked = ["shipyards", "logistics", "freight"]