Draw capture points behind fleet vessels (#50)

CapturePoint.gd left z_index at its default 0, the same layer Ship
draws on, so a ship parked inside a ring was partly swallowed by the
ring fill - exactly where the player most needs to see their own
vessels. Set z_index = -1, matching the convention every other world
object already follows (Planet, Rock, Building, Hauler, Transporter,
VesselDelivery).

Verified with a ship placed directly on a capture point in the
preview screenshot, and asserted in test_capture_points.gd.

CapturePoint.gd

@@ -28,6 +28,12 @@ var state: int = State.NEUTRAL
 
 
 func _ready() -> void:
+	# Behind the fleet and enemies, same convention as every other world
+	# object that must never occlude them (Planet.gd, Rock.gd, Building.gd,
+	# Hauler.gd, Transporter.gd, VesselDelivery.gd all set this in _ready()).
+	# "Is an enemy inside my capture radius" is the core read of this
+	# mechanic, so a ship or enemy sitting in the ring must stay fully visible.
+	z_index = -1
 	queue_redraw()