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.

BlackHolePlanetDefinition.tres

@@ -0,0 +1,14 @@
+[gd_resource type="Resource" script_class="PlanetDefinition" load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://PlanetDefinition.gd" id="1"]
+[ext_resource type="PackedScene" path="res://BlackHoleVisual.tscn" id="2"]
+
+[resource]
+script = ExtResource("1")
+definition_id = "black_hole"
+display_name = "Black hole"
+names = PackedStringArray("Event Zero", "Umbral Well", "The Sinkhole", "Lightless Reach", "Null Point")
+visual_scene = ExtResource("2")
+surface_profile_id = "void"
+resource_id = "dark_matter"
+map_color = Color(0.55, 0.15, 0.85, 1)