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>

tests/_build_menu_preview.gd

@@ -14,7 +14,9 @@ func _ready() -> void:
 	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"]
+	# Partly researched, to show the menu repacking around what is hidden.
+	GameState.unlocked = ["shipyards", "logistics"]
 	var main: Node = load("res://Main.tscn").instantiate()
 	add_child(main)
 	main.get_node("UI/BuildMenu").visible = true
+	main.get_node("BuildController")._refresh_build_status()