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.

CaptureLabel.gd

@@ -0,0 +1,32 @@
+extends Label
+
+# Polls GameState directly, same convention as ResourceCounter/GasCounter,
+# so it stays correct without anyone having to push updates into it. Hidden
+# entirely when the current planet has no capture points (e.g. no setup
+# resolved for it) rather than showing "0/0".
+
+var _was_conquered: bool = false
+
+
+func _process(_delta: float) -> void:
+	var planet_id: String = GameState.planet_id
+	var total: int = GameState.total_capture_points(planet_id)
+	if total <= 0:
+		visible = false
+		return
+	visible = true
+
+	var conquered: bool = GameState.is_planet_conquered(planet_id)
+	if conquered:
+		text = "CONQUERED"
+		if not _was_conquered:
+			_flash()
+	else:
+		text = "CAPTURED %d/%d" % [GameState.captured_point_count(planet_id), total]
+	_was_conquered = conquered
+
+
+func _flash() -> void:
+	modulate.a = 0.0
+	var tween: Tween = create_tween()
+	tween.tween_property(self, "modulate:a", 1.0, 1.2)