Transport Hub building

Forgejo #18, part 5 of #12. The building only - the vessels that do the
hauling are #19. Placeable, costed at 30, gated behind a new "Freight
Handling" research node hanging off Logistics, with its own info popup,
build-menu glyph and map marker.

Drawn as an octagonal dock ring with berth arms and a squared-off freight
core: deliberately radial-but-flat so it does not read as a small
mothership, which is the only structure with swept arms. Its stationed
transports are NeonPiece children of one rotating holder, per #10 - the
geometry is rasterised once and only the holder's rotation changes, so
the traffic costs no redraws.

Building._draw()'s match has a catch-all that renders anything
unrecognised as a ship crafter, so a missing case is silent and looks
almost right. The test checks for the hub's own _hub_traffic holder
rather than trusting that something plausible appeared - which caught
exactly that: a _ready() patch had silently not applied and the hub was
rendering through the crafter branch.

The build menu grew a third row. Every button, label and price moved:
the old row occupied 332px against a 372px pitch, and three of those do
not fit above a 1280px screen. Rows are now shorter (128px buttons, 40px
price digits) at a 290px pitch, ending at y=1176. Rendered and looked at
- the first attempt tightened the pitch alone and left every price
overlapping the row beneath it, which no headless test would have seen.

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

tests/_ingame_buildings_preview.gd

@@ -13,8 +13,9 @@ func _ready() -> void:
 	GameState.buildings.clear()
 	GameState.extractors.clear()
 	GameState.reset_formation()
-	GameState.add_building(GameState.BUILDING_MOTHERSHIP, Vector2(-230.0, 0.0))
-	GameState.add_building(GameState.BUILDING_SHIP_CRAFTER, Vector2(240.0, 0.0))
+	GameState.add_building(GameState.BUILDING_MOTHERSHIP, Vector2(-330.0, 0.0))
+	GameState.add_building(GameState.BUILDING_SHIP_CRAFTER, Vector2(140.0, 0.0))
+	GameState.add_building(GameState.BUILDING_TRANSPORT_HUB, Vector2(480.0, 0.0))
 
 	var main: Node = load("res://Main.tscn").instantiate()
 	add_child(main)
@@ -24,4 +25,4 @@ func _ready() -> void:
 	# far enough to see them whole.
 	main.get_node("Fleet").move_to(Vector2(0.0, 210.0))
 	var cam: Camera2D = main.get_node("Camera2D")
-	cam.zoom = Vector2(1.35, 1.35)
+	cam.zoom = Vector2(1.0, 1.0)