Bound individual vessel speed; stop Fleet.stop() accumulating it (#72)

Rotating the fleet and spamming a direction let vessels fall out of sync
and fly at wildly high speeds. The pivot cooldown added earlier throttled
how often the exploit could be triggered; it never touched the cause.

Fleet.stop() did `ship.velocity = ship.velocity + ship.fleet_velocity`,
compensating for the fact that Ship.velocity read zero during cruise. The
next steer refilled fleet_velocity, so every press-release cycle added a
whole cruise velocity to the ship's own, decaying only at `acceleration` in
between. Tapping faster than that bled it off stacked them without limit,
and ships desynced because each bled off at a rate set by its own distance
to its slot. Measured before this change: spamming a direction while
pivoting flew the fleet 32737 units in 3s against 784 for simply holding
the stick, with a vessel reaching 48x its own top speed.

The compensation existed because `velocity` was not the ship's real motion:
Ship snapped to its slot every tick and zeroed it, so the truth lived in a
second field and anything wanting it had to add the two together. Fixed at
that root rather than by clamping the symptom:

- fleet_velocity is now fed FORWARD into the steering instead of carried
  alongside it. The ship aims to travel at its slot's velocity plus a
  correction closing the gap, so holding formation costs no error and
  `velocity` is always true world motion.
- The snap only applies when the slot is standing still, which is the one
  case where converging asymptotically would leave a residue. While the
  slot moves the ship steers continuously - a snap there was the teleport
  that made `velocity` lie in the first place.
- A hard bound on the INTEGRATED result every tick, not merely on the
  request, so no sequence of commands can leave a vessel over its own top
  speed.
- Fleet.stop() injects nothing; it reports that the slot has halted.

tests/test_fleet_speed_bounded.gd covers it, and fails against the previous
code with the numbers above: no vessel exceeds its own top speed however
hard the direction is spammed, spamming cannot cover more ground than
holding the stick, both hold while the formation is pivoting, and the fleet
stays together.

test_space_time_distorder.gd now clears the asteroid and enemy groups at
setup, the same clean-slate idiom test_ship_lab.gd uses. Silencing the
spawners was never enough - what Main.tscn had already placed kept moving,
and losing the distorder vessel to a rock resets the multiplier correctly,
which the test then reported three stages later as a failed scene swap. It
also checks it still crews a distorder vessel before relying on that.

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

tests/test_fleet_speed_bounded.gd

@@ -0,0 +1,196 @@
+extends SceneTree
+
+# Headless gameplay test: run with
+#   godot --headless -s res://tests/test_fleet_speed_bounded.gd
+# A reported movement bug: rotating the fleet and spamming a direction let
+# vessels fall out of sync and fly at wildly high speeds - "speed of an
+# individual ship is not bounded".
+#
+# The cause was in Fleet.stop(), which compensated for Ship.velocity reading
+# zero during cruise by doing `ship.velocity += ship.fleet_velocity`. That is
+# an accumulator: each release added a whole cruise velocity, the next steer
+# refilled fleet_velocity, and tapping faster than `acceleration` could bleed
+# it off stacked them without limit. Ships desynced because each bled off at
+# a rate set by its own distance to its slot.
+#
+# Covers:
+#   A. no vessel ever exceeds its own top speed, however hard the direction
+#      is spammed - the property that was actually violated
+#   B. spamming cannot outrun an honestly-held stick: the fleet covers no
+#      more ground than cruising for the same time would
+#   C. the same holds while the formation is being pivoted, which is the
+#      case the report named
+#   D. ships stay in sync with each other rather than fanning out
+
+var game_state: Node
+var main: Node
+var fleet: Node
+var stage: int = 0
+var elapsed: float = 0.0
+var fastest_seen: float = 0.0
+var worst_spread: float = 0.0
+var start_centroid: Vector2 = Vector2.ZERO
+var spam_toggle: bool = false
+var pivot_step: int = 0
+var honest_distance: float = 0.0
+
+const SPAM_TIME: float = 3.0
+# Every ship in the test fleet is an interceptor, so one limit covers them
+# all. Two percent of slack absorbs the single-tick overshoot inherent in
+# integrating at a finite step; the bug being caught overshot by multiples,
+# not by percents.
+const SPEED_TOLERANCE: float = 1.02
+
+
+func _initialize() -> void:
+	game_state = root.get_node("GameState")
+	game_state.buildings.clear()
+	game_state.extractors.clear()
+	game_state.transporters.clear()
+	game_state.stored_vessels.clear()
+	game_state.reset_progression()
+	main = load("res://Main.tscn").instantiate()
+	root.add_child(main)
+	fleet = main.get_node("Fleet")
+
+
+func _process(delta: float) -> bool:
+	elapsed += delta
+	match stage:
+		0:
+			if elapsed < 0.2:
+				return false
+			main.get_node("EnemySpawner").set_process(false)
+			main.get_node("AsteroidField").set_process(false)
+			fleet.spawn_ships(4, Vector2.ZERO)
+
+			# --- reference: what holding the stick honestly achieves --------
+			start_centroid = fleet.get_centroid()
+			fleet.move_in_direction(Vector2(1.0, 0.0))
+			elapsed = 0.0
+			stage = 1
+			return false
+		1:
+			if elapsed < SPAM_TIME:
+				_watch_speeds()
+				return false
+			honest_distance = fleet.get_centroid().distance_to(start_centroid)
+			print("held steady for %.1fs: travelled %.1f, fastest vessel %.1f" \
+				% [SPAM_TIME, honest_distance, fastest_seen])
+			if not _check_speed_bound("while simply holding a direction"):
+				return true
+			fleet.stop()
+			fastest_seen = 0.0
+			elapsed = 0.0
+			stage = 2
+			return false
+		2:
+			# Let the coast-to-rest finish so the spam leg starts from the
+			# same standing start the honest leg did.
+			if elapsed < 1.0:
+				return false
+			start_centroid = fleet.get_centroid()
+			elapsed = 0.0
+			stage = 3
+			return false
+		3:
+			# --- A + B. tap the same direction as fast as frames allow ------
+			spam_toggle = not spam_toggle
+			if spam_toggle:
+				fleet.move_in_direction(Vector2(1.0, 0.0))
+			else:
+				fleet.stop()
+			_watch_speeds()
+			if elapsed < SPAM_TIME:
+				return false
+			var spam_distance: float = fleet.get_centroid().distance_to(start_centroid)
+			print("spammed for %.1fs: travelled %.1f, fastest vessel %.1f" \
+				% [SPAM_TIME, spam_distance, fastest_seen])
+			if not _check_speed_bound("while spamming a direction"):
+				return true
+			# Tapping releases the stick half the time, so it should cover
+			# LESS ground than holding it - certainly not more.
+			if spam_distance > honest_distance * 1.1:
+				return _fail_now("spamming a direction covered %.1f against %.1f for simply holding it - tapping is faster than flying" \
+					% [spam_distance, honest_distance])
+			fleet.stop()
+			fastest_seen = 0.0
+			elapsed = 0.0
+			stage = 4
+			return false
+		4:
+			if elapsed < 1.0:
+				return false
+			start_centroid = fleet.get_centroid()
+			elapsed = 0.0
+			stage = 5
+			return false
+		5:
+			# --- C + D. spam the direction WHILE pivoting the formation -----
+			# The reported case. A pivot re-seats every slot at once, so this
+			# is where ships are most able to fall out of step with one
+			# another.
+			spam_toggle = not spam_toggle
+			if spam_toggle:
+				fleet.move_in_direction(Vector2(1.0, 0.0))
+			else:
+				fleet.stop()
+			pivot_step += 1
+			_pivot_formation(PI / 2.0 if pivot_step % 2 == 0 else -PI / 2.0)
+			_watch_speeds()
+			if elapsed < SPAM_TIME:
+				return false
+			var pivot_distance: float = fleet.get_centroid().distance_to(start_centroid)
+			print("spammed with pivots for %.1fs: travelled %.1f, fastest vessel %.1f, worst pair spread %.1f" \
+				% [SPAM_TIME, pivot_distance, fastest_seen, worst_spread])
+			if not _check_speed_bound("while spamming a direction and pivoting"):
+				return true
+			if pivot_distance > honest_distance * 1.1:
+				return _fail_now("spamming with pivots covered %.1f against %.1f for simply holding a direction" \
+					% [pivot_distance, honest_distance])
+			# Ships must remain a fleet. The formation is a wedge a couple of
+			# hundred units across; a desync flings one vessel far beyond it.
+			if worst_spread > 900.0:
+				return _fail_now("vessels fell %.1f apart - the fleet came apart rather than moving together" % worst_spread)
+			print("no vessel outran its own top speed, and none broke formation")
+
+			print("TEST_FLEET_SPEED_BOUNDED: PASS")
+			quit(0)
+			return true
+	return false
+
+
+func _watch_speeds() -> void:
+	var boost: float = game_state.fleet_speed_multiplier
+	for ship in fleet.ships:
+		if not is_instance_valid(ship):
+			continue
+		fastest_seen = maxf(fastest_seen, ship.velocity.length() / (ship.max_speed * boost))
+		for other in fleet.ships:
+			if is_instance_valid(other):
+				worst_spread = maxf(worst_spread, ship.position.distance_to(other.position))
+
+
+# fastest_seen is carried as a RATIO of each vessel's own limit, so a mixed
+# fleet with different top speeds would be judged per vessel rather than
+# against a single global number.
+func _check_speed_bound(context: String) -> bool:
+	if fastest_seen > SPEED_TOLERANCE:
+		_fail_now("a vessel reached %.2fx its own top speed %s - individual speed is not bounded" \
+			% [fastest_seen, context])
+		return false
+	return true
+
+
+func _pivot_formation(angle: float) -> void:
+	var rotated: Array = []
+	for offset in game_state.formation:
+		rotated.append(Vector2(offset).rotated(angle))
+	game_state.formation = rotated
+	fleet.apply_formation()
+
+
+func _fail_now(message: String) -> bool:
+	print("TEST_FLEET_SPEED_BOUNDED: FAIL (%s)" % message)
+	quit(1)
+	return true