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.

WorldGen.gd

@@ -16,6 +16,12 @@ const NOISE_SEED: int = 1337
 const ASTEROID_FIELD_THRESHOLD: float = 0.15
 const PLANET_THRESHOLD: float = 0.33
 const GAS_GIANT_THRESHOLD: float = 0.55
+# A flat per-planet-chunk probability rather than another noise field: gas
+# giants already cover every SPACE-biome planet chunk (see GAS_GIANT_THRESHOLD
+# above), so making black holes noticeably rarer just needs a small, uniform
+# chance independent of biome - not a second smooth region to tune against
+# the first.
+const BLACK_HOLE_CHANCE: float = 0.05
 
 # A predictable early gas source for play-testing. The origin chunk is
 # deterministically normal space. Its explicit in-chunk position puts it
@@ -84,10 +90,20 @@ static func planet_data_at_chunk(chunk: Vector2i) -> Dictionary:
 	var local_pos: Vector2 = NEARBY_GAS_GIANT_LOCAL_POS if guaranteed_nearby_giant \
 		else Vector2(rng.randf() * CHUNK_SIZE, rng.randf() * CHUNK_SIZE)
 	var shape_seed: int = rng.randi()
+	# Drawn unconditionally, in the same fixed position every call,
+	# regardless of biome or the gas-giant roll below - so adding this
+	# roll doesn't shift which random values any LATER draw (planet_name,
+	# below) consumes for chunks that would have existed before it.
+	var black_hole_roll: float = rng.randf()
 	# A stable per-chunk roll makes gas giants persistent landmarks rather
 	# than a different resource whenever the scene reloads.
 	var gas_giant: bool = guaranteed_nearby_giant or biome_at_chunk(chunk) == Biome.SPACE
 	var definition_id: String = PLANET_DEFINITIONS.GAS_GIANT_ID if gas_giant else PLANET_DEFINITIONS.ROCKY_ID
+	# A rare third outcome, seeded the same deterministic way as everything
+	# else here. Never overrides the guaranteed early gas giant landmark -
+	# that chunk's whole purpose is to always be the same reachable source.
+	if not guaranteed_nearby_giant and black_hole_roll < BLACK_HOLE_CHANCE:
+		definition_id = PLANET_DEFINITIONS.BLACK_HOLE_ID
 	var definition: Resource = PLANET_DEFINITIONS.get_definition(definition_id)
 	var names: PackedStringArray = definition.names
 	var planet_name: String = names[rng.randi() % names.size()]