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>

GameState.gd

@@ -101,9 +101,13 @@ const BUILDING_COMMAND_CENTER: String = "command_center"
 # Extractors are not placed in space like the others - they are built ON a
 # planet's surface and tracked separately (see `extractors`).
 const BUILDING_EXTRACTOR: String = "extractor"
+# Hosts the transport vessels that supply other buildings (#18). Built in
+# space like the rest; it is the transporters (#19) that do the moving.
+const BUILDING_TRANSPORT_HUB: String = "transport_hub"
 
 const COMMAND_CENTER_COST: int = 15
 const EXTRACTOR_COST: int = 25
+const TRANSPORT_HUB_COST: int = 30
 
 var buildings: Array = []
 
@@ -136,6 +140,11 @@ const SKILL_TREE: Array = [
 		"x": 0.82, "unlocks": BUILDING_COMMAND_CENTER,
 		"desc": "Build command centers, where the fleet formation is arranged.",
 	},
+	{
+		"id": "freight", "name": "Freight Handling", "cost": 40, "requires": "logistics",
+		"x": 0.18, "unlocks": BUILDING_TRANSPORT_HUB,
+		"desc": "Build transport hubs, which supply your other buildings with ore.",
+	},
 	{
 		"id": "wing", "name": "Wing Doctrine", "cost": 30, "requires": "coordination",
 		"x": 0.82, "fleet_bonus": 1,