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.

PlanetSurfaceController.gd

@@ -44,7 +44,7 @@ func _compose_surface(profile: Resource) -> void:
 	atmosphere.set_shader_parameter("base_color", profile.base_color)
 	atmosphere.set_shader_parameter("crater_color", profile.crater_color)
 	atmosphere.set_shader_parameter("highlight_color", profile.highlight_color)
-	for caption_path in ["../UI/BankCaption", "../UI/FleetCaption"]:
+	for caption_path in ["../UI/BankCaption", "../UI/FleetCaption", "../UI/CaptureLabel"]:
 		var caption: Label = get_node(caption_path)
 		caption.add_theme_color_override("font_color", profile.caption_color)
 		caption.add_theme_color_override("font_outline_color", profile.caption_outline_color)