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.

EnemyStandoffComponent.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://EnemyStandoffComponent.gd" id="1"]
+
+[node name="EnemyStandoffComponent" type="Node"]
+script = ExtResource("1")