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.

tests/_gas_bomb_aim_preview.gd

@@ -39,5 +39,5 @@ func _ready() -> void:
 	for offset in [Vector2(-65, 0), Vector2(45, -45), Vector2(55, 65)]:
 		var enemy: Node = load("res://Enemy.tscn").instantiate()
 		enemy.position = skill.aim_target + offset
-		enemy.speed = 0.0
 		main.add_child(enemy)
+		enemy.speed = 0.0   # after add_child - _ready() sets it from the definition