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>

WorldMapContent.gd

@@ -10,6 +10,7 @@ const WORLD_TO_LOCAL: float = 0.04
 const MOTHERSHIP_MARKER_COLOR: Color = Color(0.45, 0.9, 0.55)
 const CRAFTER_MARKER_COLOR: Color = Color(1.0, 0.72, 0.25)
 const COMMAND_MARKER_COLOR: Color = Color(0.55, 0.6, 1.0)
+const TRANSPORT_HUB_MARKER_COLOR: Color = Color(0.95, 0.55, 0.85)
 
 # A structure built on a planet's surface (see GameState.extractors) is not
 # itself a map marker - it is a small badge on the existing planet marker,
@@ -50,6 +51,8 @@ func _building_marker_color(type: String) -> Color:
 			return MOTHERSHIP_MARKER_COLOR
 		GameState.BUILDING_SHIP_CRAFTER:
 			return CRAFTER_MARKER_COLOR
+		GameState.BUILDING_TRANSPORT_HUB:
+			return TRANSPORT_HUB_MARKER_COLOR
 		_:
 			return COMMAND_MARKER_COLOR
 
@@ -115,6 +118,8 @@ func _draw() -> void:
 				_draw_marker_polygon(_regular_polygon(marker.position, 8.0, 6, PI / 6.0), marker.color)
 			GameState.BUILDING_SHIP_CRAFTER:
 				_draw_marker_polygon(_regular_polygon(marker.position, 7.0, 4, 0.0), marker.color)
+			GameState.BUILDING_TRANSPORT_HUB:
+				_draw_marker_polygon(_regular_polygon(marker.position, 7.5, 8, PI / 8.0), marker.color)
 			_:
 				_draw_marker_polygon(_regular_polygon(marker.position, 6.0, 4, PI / 4.0), marker.color)