Fix the stop stutter, and hide unresearched buildings from the menu

Forgejo #13: releasing the stick paused, then moved again before
settling. stop() aims each ship at a braking point ahead of it, but the
ship's own velocity was ~0 - the anchor had been carrying it - so it
started from REST, accelerated into its own stopping distance, and
halted a second time. Measured: 0 -> 45 -> 90 -> 135 -> 180 -> brake.
stop() now hands the fleet's motion to the ship, so the release is one
continuous decelerating coast (222 -> 0) to the identical endpoint.
test_joystick_control now asserts the speed one frame after release is a
real fraction of cruise, and never rises afterwards.

Forgejo #21: unresearched structures are no longer listed greyed out.
They are hidden, and the menu repacks around what is left - a fixed grid
with entries hidden would otherwise leave holes where they used to be.
An entry alone on its row is centred rather than hanging in the left
column. Researched-but-not-usable is still shown greyed: an extractor
with no planet under the fleet is "not now", not "not yet".

This reverses a documented decision (the old comment argued that always
listing everything stops the player wondering whether the game has it at
all), so test_skill_tree's matching assertion was updated from
greyed-but-listed to hidden. The gate itself is unchanged and still
refuses a build forced past the menu.

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

BuildController.gd

@@ -28,6 +28,18 @@ extends Node2D
 @onready var build_status: Label = get_node("../UI/BuildMenu/StatusLabel")
 @onready var build_extractor_button: IconButton = get_node("../UI/BuildMenu/ExtractorButton")
 @onready var build_hub_button: IconButton = get_node("../UI/BuildMenu/TransportHubButton")
+@onready var build_menu_entries: Array = [
+	[GameState.BUILDING_MOTHERSHIP, build_mothership_button,
+		get_node("../UI/BuildMenu/MothershipLabel"), get_node("../UI/BuildMenu/MothershipCost")],
+	[GameState.BUILDING_SHIP_CRAFTER, build_crafter_button,
+		get_node("../UI/BuildMenu/CrafterLabel"), get_node("../UI/BuildMenu/CrafterCost")],
+	[GameState.BUILDING_COMMAND_CENTER, build_command_button,
+		get_node("../UI/BuildMenu/CommandLabel"), get_node("../UI/BuildMenu/CommandCost")],
+	[GameState.BUILDING_EXTRACTOR, build_extractor_button,
+		get_node("../UI/BuildMenu/ExtractorLabel"), get_node("../UI/BuildMenu/ExtractorCost")],
+	[GameState.BUILDING_TRANSPORT_HUB, build_hub_button,
+		get_node("../UI/BuildMenu/TransportHubLabel"), get_node("../UI/BuildMenu/TransportHubCost")],
+]
 @onready var planet_field: Node2D = get_node("../PlanetField")
 @onready var info_popup: Control = get_node("../UI/BuildMenu/InfoPopup")
 @onready var info_title: Label = get_node("../UI/BuildMenu/InfoPopup/Panel/TitleLabel")
@@ -185,6 +197,7 @@ func _process(delta: float) -> void:
 		# a planet lights the option up without reopening it.
 		build_extractor_button.enabled = _planet_under_fleet() != null \
 			and GameState.is_building_unlocked(GameState.BUILDING_EXTRACTOR)
+		build_extractor_button.queue_redraw()
 
 	if craft_menu.visible:
 		# Live, so a delivery arriving while the menu is open visibly works
@@ -266,14 +279,58 @@ func _planet_under_fleet() -> Node2D:
 	return planet_field.get_planet_fleet_is_on()
 
 
+# Grid geometry, shared with the layout below. Rows of two, and the menu is
+# rebuilt from whatever is currently researched (#21) rather than being a
+# fixed arrangement with holes punched in it.
+const MENU_ROW_TOPS: Array = [-300.0, -10.0, 280.0]
+const MENU_COL_X: float = 140.0
+const MENU_BUTTON_HALF: float = 64.0
+const MENU_BUTTON_H: float = 128.0
+const MENU_LABEL_HALF: float = 150.0
+const MENU_LABEL_H: float = 66.0
+const MENU_COST_HALF: float = 60.0
+const MENU_COST_H: float = 52.0
+
+
+func _place_entry(node: Control, cx: float, top: float, half: float, height: float) -> void:
+	node.offset_left = cx - half
+	node.offset_right = cx + half
+	node.offset_top = top
+	node.offset_bottom = top + height
+
+
+func _layout_build_menu() -> void:
+	# Structures the player has not researched are hidden outright (#21), and
+	# what remains is repacked so there are no gaps where they used to be.
+	# This reverses the earlier decision to always list everything greyed
+	# out; an option you cannot explain is apparently worse than one you
+	# cannot see yet.
+	var shown: Array = []
+	for entry in build_menu_entries:
+		var visible: bool = GameState.is_building_unlocked(entry[0])
+		entry[1].visible = visible
+		entry[2].visible = visible
+		entry[3].visible = visible
+		if visible:
+			shown.append(entry)
+
+	for i in range(shown.size()):
+		var row: int = i / 2
+		# Centred when it is the only one on its row, rather than hanging in
+		# the left column looking like something is missing beside it.
+		var alone: bool = (i == shown.size() - 1) and (i % 2 == 0)
+		var cx: float = 0.0 if alone else (-MENU_COL_X if i % 2 == 0 else MENU_COL_X)
+		var top: float = MENU_ROW_TOPS[mini(row, MENU_ROW_TOPS.size() - 1)]
+		_place_entry(shown[i][1], cx, top, MENU_BUTTON_HALF, MENU_BUTTON_H)
+		_place_entry(shown[i][2], cx, top + 134.0, MENU_LABEL_HALF, MENU_LABEL_H)
+		_place_entry(shown[i][3], cx, top + 204.0, MENU_COST_HALF, MENU_COST_H)
+
+
 func _refresh_build_status() -> void:
-	# Every option is always listed, greyed when it cannot be used - showing
-	# them only sometimes would leave the player wondering whether the game
-	# has them at all. Greyed buttons still emit `pressed`, so each one can
-	# explain itself in the info popup.
-	build_crafter_button.enabled = GameState.is_building_unlocked(GameState.BUILDING_SHIP_CRAFTER)
-	build_command_button.enabled = GameState.is_building_unlocked(GameState.BUILDING_COMMAND_CENTER)
-	build_hub_button.enabled = GameState.is_building_unlocked(GameState.BUILDING_TRANSPORT_HUB)
+	_layout_build_menu()
+	# Researched but not usable right here is still shown, greyed: an
+	# extractor needs a planet under the fleet, which is a "not now" rather
+	# than a "not yet".
 	build_extractor_button.enabled = _planet_under_fleet() != null \
 		and GameState.is_building_unlocked(GameState.BUILDING_EXTRACTOR)