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.

Fleet.gd

@@ -32,6 +32,10 @@ var steer_direction: Vector2 = Vector2.ZERO
 
 
 func _ready() -> void:
+	# Lets anything resolve "the fleet" without a hardcoded ../Fleet sibling
+	# path - enemies parented under different hosts (e.g. a planet surface's
+	# SurfaceFeatures) need this to find their target.
+	add_to_group("player_fleet")
 	# Researching a vessel upgrade improves the ships already flying, not
 	# just the next one built (#5).
 	GameState.vessel_upgraded.connect(_on_vessel_upgraded)