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/_black_hole_space_preview.gd

@@ -0,0 +1,23 @@
+extends Node2D
+
+# Preview, not a test - a black hole planet as seen in space, next to a
+# rocky planet for scale/contrast.
+#
+#   bash tests/screenshot.sh res://tests/_black_hole_space_preview.tscn /tmp/black_hole.png 15 720x1280
+
+
+func _ready() -> void:
+	GameState.extractors.clear()
+	GameState.buildings.clear()
+
+	_spawn("Event Zero", "black_hole", Vector2(360.0, 420.0), 12345)
+	_spawn("Dustworn", "rocky", Vector2(360.0, 950.0), 4242)
+
+
+func _spawn(planet_name: String, definition_id: String, at: Vector2, shape_seed: int) -> void:
+	var planet: Node2D = load("res://Planet.tscn").instantiate()
+	planet.planet_name = planet_name
+	planet.definition_id = definition_id
+	planet.shape_seed = shape_seed
+	planet.position = at
+	add_child(planet)