feat: lit/library-save creates library entries like Divi (E4)

Reuses NewDraft for insert and rollback (P6), requires edit_global_library
for scope global (P7), refuses opaque and global roots before wrapping (P11)
and checks the node with StoredNodeGuard. The source page is never written.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/Divi/DiviIntegrationTest.php

@@ -58,7 +58,7 @@ final class DiviIntegrationTest extends TestCase
         self::assertSame('Divi-Bibliothek', $library->label());
         self::assertFalse($library->isAvailable(), 'no Divi classes in unit tests');
         self::assertSame(
-            ['lit/library-list'],
+            ['lit/library-list', 'lit/library-save'],
             array_map(static fn (Ability $ability): string => $ability->name(), $library->abilities())
         );
         foreach ($library->abilities() as $ability) {