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.
