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.

GameState.gd

@@ -1107,6 +1107,10 @@ func captured_point_count(planet_key: String) -> int:
 	return count
 
 
+func total_capture_points(planet_key: String) -> int:
+	return capture_state(planet_key).get("captured", []).size()
+
+
 func _process(delta: float) -> void:
 	# Runs in the autoload, so mining and transport continue while the player
 	# is on a planet's surface or nowhere near either.