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.

PlanetDefinitions.gd

@@ -3,10 +3,12 @@ extends RefCounted
 
 const ROCKY_ID: String = "rocky"
 const GAS_GIANT_ID: String = "gas_giant"
+const BLACK_HOLE_ID: String = "black_hole"
 const DEFAULT_DEFINITION_ID: String = ROCKY_ID
 const DEFINITIONS: Dictionary = {
 	ROCKY_ID: preload("res://RockyPlanetDefinition.tres"),
 	GAS_GIANT_ID: preload("res://GasGiantDefinition.tres"),
+	BLACK_HOLE_ID: preload("res://BlackHolePlanetDefinition.tres"),
 }