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/_build_menu_preview.gd

@@ -0,0 +1,20 @@
+extends Node2D
+
+# Preview, not a test. Opens the build menu itself, so the three-row layout
+# added for the Transport Hub (#18) can be checked for fit on the real phone
+# viewport - every button, label and price moved, and a bad offset is
+# invisible to a headless test.
+#
+#   bash tests/screenshot.sh res://tests/_build_menu_preview.tscn /tmp/menu.png 20 720x1280
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.rocks = 500
+	# Everything researched, so nothing is greyed out and the whole menu is
+	# legible at full strength.
+	GameState.unlocked = ["shipyards", "logistics", "freight", "coordination"]
+	var main: Node = load("res://Main.tscn").instantiate()
+	add_child(main)
+	main.get_node("UI/BuildMenu").visible = true