refactor: narrow over-exposed API (L2)

Composer::autoLabel is private (tested through compose()); the
DiviComposition getters used only by tests are marked @internal;
Limits::snapshotCount is documented as reserved; the ShortformWriter test
seam is named $modulesForTests.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Divi/Compose/Composer.php

@@ -232,7 +232,7 @@ final class Composer
     }
 
     /** Admin label for a new module: "<Abschnitt> – <Modultitel>" or the title alone. */
-    public function autoLabel(string $name, string $context): string
+    private function autoLabel(string $name, string $context): string
     {
         $title = $this->catalog->title($name);