Add capture points and the planet conquest loop (#43)

CapturePoint tracks neutral/blocked/capturing/captured state by polling
proximity each frame (a ship in capture_radius, no enemy within
enemy_clear_radius), draining gradually rather than resetting when
interrupted, and calling GameState.mark_point_captured() on completion.
PlanetSetupHost now instantiates the real CapturePoint scene per
authored point (restoring already-captured state immediately on
re-landing) and spawns each setup's authored enemies, replacing the
placeholder markers from #42. CaptureLabel is a small self-polling HUD
(same convention as ResourceCounter) showing "CAPTURED n/m" or
"CONQUERED" with a one-shot fade on completion, themed per surface
profile alongside the existing bank/fleet captions.

test_planet_atmosphere_transition.gd's "no enemies on a landed
surface" assumption no longer holds now that setups can place guard
enemies by design - narrowed the check to what it actually needs to
prove: no space-side EnemySpawner/AsteroidField node leaks across the
scene swap.

Part of the #40 conquest-mechanic breakdown.

tests/_capture_points_preview.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://tests/_capture_points_preview.gd" id="1"]
+
+[node name="CapturePointsPreview" type="Node2D"]
+script = ExtResource("1")