Add black hole planet and dark matter resource (#55)

A rare, deterministically-seeded landable body yielding a fourth
resource, dark matter, obtainable only through an extractor - never a
surface pickup. Reuses every existing generic system (resource/planet
definitions, surface profiles, extractor/transporter pipeline) with no
type-switch branching.

Also fixes a latent conquest-gating gap: a surface profile with no
authored PlanetSetup (like the black hole's "void") never registered
capture points at all, permanently blocking extractor placement.
register_capture_points() now treats zero required points as vacuously
conquered, and PlanetSurfaceController registers that zero-point state
explicitly when no setup exists.

tests/_void_surface_preview.gd

@@ -0,0 +1,15 @@
+extends Node2D
+
+# Preview, not a test - a black hole's landed "void" surface: no craters, no
+# resource pickups, nothing but the empty field itself.
+#
+#   bash tests/screenshot.sh res://tests/_void_surface_preview.tscn /tmp/void.png 15 720x1280
+
+
+func _ready() -> void:
+	GameState.planet_name = "Event Zero"
+	GameState.planet_definition_id = "black_hole"
+	GameState.surviving_ship_count = 3
+	var surface: Node = load("res://PlanetSurface.tscn").instantiate()
+	surface.get_node("SurfaceController").fade_duration = 0.0
+	add_child(surface)