Refine boss reveal and dash motion

tests/test_energy_propulser_dash.gd

@@ -65,6 +65,16 @@ func _process(delta: float) -> bool:
 				return _fail("first dash did not consume %d energy" % DASH_COST)
 			if first_component.is_ready():
 				return _fail("the Propulser that dashed did not enter cooldown")
+			# The burst should cover substantially more than a linear quarter of
+			# its distance at a quarter of its duration, then ease into its stop.
+			fleet.set_process(false)
+			fleet._advance_dash(fleet.dash_duration * 0.25)
+			var early_distance: float = fleet.get_centroid().distance_to(first_origin)
+			if early_distance <= DASH_DISTANCE * 0.45 \
+					or early_distance >= DASH_DISTANCE * 0.7:
+				return _fail("dash has no useful ease-out curve (quarter-time distance %.1f)" \
+					% early_distance)
+			fleet.set_process(true)
 			dash._process(0.0)
 			if not main.get_node("UI/DashButton").enabled:
 				return _fail("one cooling Propulser disabled a second ready one")