Gate enemies behind a detection range instead of instant omniscient pursuit (#69)

Chasers (EnemyPursuitComponent) and snipers (EnemyStandoffComponent)
used to beeline/close in from anywhere on the map the instant they
spawned - no matter how far the fleet was, which read as omniscient
rather than alert. Both now sit idle until the fleet is within their
own detection_range. Artillery's slow drift is gated the same way, but
reuses its own existing fire_range rather than a new tunable - outside
its firing envelope it has nothing to be drifting toward yet.

The enemy mothership deliberately keeps the opposite behaviour: it is
meant to read as a persistent, unavoidable stalking threat, not a
local sentry, so it gets its own EnemyPursuitComponentMothership.tscn
variant with an effectively unlimited detection_range - the same
scene-variant pattern EnemySpawnerComponentChaser/Artillery already
use for per-definition property overrides on a shared component script.

EnemyArtilleryComponent.gd

@@ -56,6 +56,11 @@ func _drift(enemy: Node2D, delta: float) -> void:
 	if fleet == null or fleet.ships.is_empty():
 		return
 	var to_fleet: Vector2 = fleet.get_centroid() - enemy.position
+	# #69: gated by the same fire_range it already shells within, rather
+	# than a second tunable that could drift out of sync - outside its own
+	# firing envelope it has nothing to be drifting toward yet.
+	if to_fleet.length() > fire_range:
+		return
 	if to_fleet.length() < 1.0:
 		return
 	enemy.position += to_fleet.normalized() * drift_speed * delta