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()
 
 

tests/_capture_points_preview.gd

@@ -28,7 +28,10 @@ func _ready() -> void:
 
 	# Lay the three points out in a compact, clearly-separated triangle for
 	# the screenshot, rather than the real (far-flung) authored positions.
-	var layout: Array = [Vector2(300, 150), Vector2(-300, 150), Vector2(0, -280)]
+	# Point 0 sits at the origin - exactly where the fleet spawns - so the
+	# screenshot also proves the #50 z_index fix: the ring must not occlude
+	# the ships parked on top of it.
+	var layout: Array = [Vector2(0, 0), Vector2(-300, 150), Vector2(300, -200)]
 	for i in range(mini(points.size(), layout.size())):
 		points[i].position = layout[i]
 

tests/test_capture_points.gd

@@ -197,6 +197,8 @@ func _process(delta: float) -> bool:
 			# through the build UI here to avoid duplicating that coverage.
 			if not game_state.is_planet_conquered(planet_id):
 				return _fail("single-point planet was not marked conquered")
+			if point.z_index >= 0:
+				return _fail("capture point does not draw behind the fleet/enemies (z_index=%d)" % point.z_index)
 
 			print("TEST_CAPTURE_POINTS: PASS")
 			quit(0)