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.

EnemyStandoffComponent.gd

@@ -11,6 +11,10 @@ var capability_id: String = "standoff"
 
 @export var preferred_range: float = 620.0
 @export var retreat_range_factor: float = 0.75
+# #69: kept comfortably beyond preferred_range so it still has room to close
+# in before settling at stand-off distance, but the fleet has to actually be
+# somewhere on the board first - see EnemyPursuitComponent's identical fix.
+@export var detection_range: float = 1200.0
 
 var velocity: Vector2 = Vector2.ZERO
 
@@ -22,7 +26,7 @@ func tick(delta: float) -> void:
 		return
 	var to_target: Vector2 = target.position - enemy.position
 	var distance: float = to_target.length()
-	if distance < 1.0:
+	if distance < 1.0 or distance > detection_range:
 		return
 	var direction: Vector2 = to_target / distance
 	var retreat_range: float = preferred_range * retreat_range_factor