Add standoff sniper enemy (#45)

EnemyStandoffComponent holds a preferred distance from the nearest
player ship instead of ramming (advance beyond it, retreat inside
retreat_range_factor of it, drift laterally in the comfortable band
between) - never paired with a pursuit component. EnemyRangedWeaponComponent
mirrors VesselWeaponComponent for an enemy: fires an EnemyProjectile
aimed at the target's CURRENT position rather than led, so a moving
ship can dodge it (projectile_speed 260 vs the interceptor's 460
max_speed). Registered in EnemyDefinitions so it's placeable from a
planet setup by enemy_id.

test_sniper_enemy.gd went through a few iterations to get its own
timing right: physics ticks can run ahead of a SceneTree test's own
_process() clock under this device's load, so the cooldown-violation
check reads the component's own fire_timer directly (a rising value
means it refired) instead of comparing projectile counts against a
wall-clock wait, and manually repositioned ships need their
move_target re-pinned or Ship's own steering silently drives them back
to the stale target.

Part of the #40 conquest-mechanic breakdown.

tests/_sniper_vs_chaser_preview.gd

@@ -0,0 +1,76 @@
+extends Node2D
+
+# Setup is deferred to the first _process() tick rather than done in
+# _ready(): PlanetAtmosphereController auto-spawns a 5-ship default fleet
+# in ITS _ready() on any fresh (non-returning) Main.tscn load, and since
+# this scene is itself added to the tree from inside _screenshot_tool.gd's
+# own _initialize() (a nested, not-yet-settled context), there's no
+# guaranteed ordering between the two _ready() calls - clearing the fleet
+# here could run BEFORE that auto-spawn and then get silently overwritten
+# by it. Waiting one frame guarantees it has already run first.
+#
+# Even after clearing, physics ticks can run several times before this
+# script's own next _process() tick (softpipe rendering is slow, and
+# physics is decoupled from it), so an auto-spawned ship can still get one
+# shot off in flight toward these enemies before the clearing code above
+# ever runs. Pausing the whole tree once everything is placed (last thing
+# this script does) freezes any such shot in transit for good, rather than
+# letting it arrive a few physics ticks later while the screenshot is
+# still waiting out its frame count.
+
+var main: Node
+var setup_done: bool = false
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.transporters.clear()
+	main = load("res://Main.tscn").instantiate()
+	add_child(main)
+	main.get_node("BackgroundLayer").visible = false
+	main.get_node("AsteroidField").visible = false
+	main.get_node("PlanetField").visible = false
+	main.get_node("BuildingField").visible = false
+	main.get_node("EnemySpawner").set_process(false)
+	main.get_node("AsteroidField").set_process(false)
+
+
+func _process(_delta: float) -> void:
+	if setup_done:
+		return
+	setup_done = true
+
+	var fleet: Node = main.get_node("Fleet")
+	for ship in fleet.ships:
+		if is_instance_valid(ship):
+			ship.set_physics_process(false)
+	fleet.spawn_ships(0, Vector2.ZERO)
+	# spawn_ships(0, ...) still isn't a truly empty fleet: it falls back to
+	# one default interceptor whenever the requested composition would
+	# otherwise be empty (Fleet.gd's own "the fleet is never really empty"
+	# rule). Clear that forced ship too, and halt it the same way.
+	for ship in fleet.ships:
+		if is_instance_valid(ship):
+			ship.set_physics_process(false)
+			ship.queue_free()
+	fleet.ships.clear()
+	for projectile in get_tree().get_nodes_in_group("player_projectiles"):
+		projectile.queue_free()
+
+	var chaser: Node = load("res://Enemy.tscn").instantiate()
+	chaser.enemy_id = "chaser"
+	chaser.position = Vector2(-160, -80)
+	main.add_child(chaser)
+	chaser.speed = 0.0
+
+	var sniper: Node = load("res://Enemy.tscn").instantiate()
+	sniper.enemy_id = "sniper"
+	sniper.position = Vector2(160, -80)
+	main.add_child(sniper)
+	sniper.speed = 0.0
+
+	var camera: Camera2D = main.get_node("Camera2D")
+	camera.zoom = Vector2(1.4, 1.4)
+
+	get_tree().paused = true