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/Limits.php

@@ -46,6 +46,7 @@ final class Limits
         return $this->all()['log_retention_days'];
     }
 
+    /** Reserved for design-system snapshots (Etappe 3); LimitsTab keeps the field hidden until then. */
     public function snapshotCount(): int
     {
         return $this->all()['snapshot_count'];