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/Pages/DiviComposition.php

@@ -61,7 +61,11 @@ class DiviComposition
         return $bound;
     }
 
-    /** The one checker of $variable(…)$ values of this composition, with the entry's live map (S22, S31). */
+    /**
+     * The one checker of $variable(…)$ values of this composition, with the entry's live map (S22, S31).
+     *
+     * @internal Public for tests; code uses cleaner() / composer().
+     */
     public function references(): VariableRef
     {
         return $this->references ??= new VariableRef(
@@ -70,7 +74,11 @@ class DiviComposition
         );
     }
 
-    /** The one Placeholders instance of this composition (ruling S31). */
+    /**
+     * The one Placeholders instance of this composition (ruling S31).
+     *
+     * @internal Public for tests; code uses attrWriter() / composer().
+     */
     public function placeholders(): Placeholders
     {
         return $this->placeholders ??= new Placeholders($this->design->read());
@@ -100,6 +108,7 @@ class DiviComposition
         );
     }
 
+    /** @internal Public for tests; code uses composer() / editor(). */
     public function rules(): StructureRules
     {
         return new StructureRules($this->modules->catalog());