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/test_joystick_control.gd

@@ -21,6 +21,11 @@ var elapsed: float = 0.0
 var start_centroid: Vector2
 var moving_centroid: Vector2
 var stop_check_frames: int = 0
+# Release-coast tracking (Forgejo #13).
+var released: bool = false
+var speed_at_release: float = 0.0
+var previous_speed: float = 0.0
+var accelerated_after_release: bool = false
 
 
 func _initialize() -> void:
@@ -54,8 +59,27 @@ func _release(local: Vector2) -> void:
 	joystick.handle_touch(ev)
 
 
+func _max_ship_speed() -> float:
+	var top: float = 0.0
+	for ship in fleet.ships:
+		top = maxf(top, ship.velocity.length())
+	return top
+
+
 func _process(delta: float) -> bool:
 	elapsed += delta
+
+	# Releasing the stick must be ONE continuous decelerating coast. It used
+	# to stop dead and then accelerate into its own stopping distance,
+	# halting a second time - felt in game as a pause followed by a small
+	# extra lurch (#13). The ships were being aimed at a braking point ahead
+	# while their own velocity was ~0, because the anchor had been carrying
+	# them, so they started from rest.
+	if released:
+		var now: float = _max_ship_speed()
+		if now > previous_speed + 1.0:
+			accelerated_after_release = true
+		previous_speed = now
 	match stage:
 		0:
 			main_scene.get_node("AsteroidField").set_process(false)
@@ -104,6 +128,9 @@ func _process(delta: float) -> bool:
 				return true
 
 			_release(joystick.size / 2.0)
+			released = true
+			speed_at_release = _max_ship_speed()
+			previous_speed = speed_at_release
 			if joystick.direction != Vector2.ZERO:
 				print("TEST_JOYSTICK_CONTROL: FAIL (direction not cleared on release)")
 				quit(1)
@@ -119,6 +146,20 @@ func _process(delta: float) -> bool:
 			var speed: float = 0.0
 			for ship in fleet.ships:
 				speed = max(speed, ship.velocity.length())
+			# Carrying the fleet's cruise speed, not starting from a standstill.
+			if speed_at_release < fleet.cruise_speed * 0.5:
+				print("speed one frame after release: %.1f (cruising at %.1f)" % [
+					speed_at_release, fleet.cruise_speed
+				])
+				print("TEST_JOYSTICK_CONTROL: FAIL (the fleet stopped dead on release instead of coasting)")
+				quit(1)
+				return true
+			if accelerated_after_release:
+				print("TEST_JOYSTICK_CONTROL: FAIL (the fleet sped up again after release - pause then lurch, #13)")
+				quit(1)
+				return true
+			print("release coast: %.1f -> 0 with no re-acceleration" % speed_at_release)
+
 			var stopped: bool = speed < 1.0
 			# Not merely "moved forward at all": the fleet cruises at
 			# cruise_speed and brakes at Ship.acceleration, so the coast is