Add enemy foundation: definitions, components, ranged damage (#44)

Enemy.gd now resolves stats/hull/color from EnemyDefinition resources
and composes behaviour from capability components (EnemyPursuitComponent
holds today's chase-the-nearest-ship logic), the same shape Ship.gd
already uses for vessels. Fleet lookups go through a "player_fleet"
group instead of a hardcoded ../Fleet sibling path, so an enemy parented
anywhere (e.g. under a planet surface's SurfaceFeatures) still finds it.

Also closes a real gap: enemies previously had no way to damage the
fleet at range at all (Ship only reacted to physical contact,
Projectile.gd was player-only). Ship.take_damage() is now a proper
entry point - contact damage routes through it unchanged - and
EnemyProjectile lets an enemy component fire at player_ships. Both
ranged enemy archetypes in #45/#46 build on this.

Part of the #40 conquest-mechanic breakdown.

EnemyProjectile.tscn

@@ -0,0 +1,12 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://EnemyProjectile.gd" id="1"]
+
+[sub_resource type="CircleShape2D" id="1"]
+radius = 4.0
+
+[node name="EnemyProjectile" type="Area2D"]
+script = ExtResource("1")
+
+[node name="CollisionShape2D" type="CollisionShape2D" parent="."]
+shape = SubResource("1")