feat: lit/divi-insert and lit/divi-update

All-or-nothing at ability level (S39), duplicate ids refused (S14),
set and attrs on the same path refused (S38), invalid_attr_path error.

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

tests/Unit/Integrations/Divi/Compose/ComposerTest.php

@@ -183,6 +183,26 @@ final class ComposerTest extends TestCase
         self::assertStringContainsString('nur lesbar', $readOnly['errors']['changes[2].id']);
     }
 
+    /** Ruling S38: one change may not reach the same attribute path through set and attrs. */
+    public function testUpdateRefusesSetAndAttrsOnTheSamePath(): void
+    {
+        $block = Block::create('divi/button', ['button' => ['innerContent' => ['desktop' => ['value' => ['text' => 'Alt']]]]]);
+
+        $result = ComposeFixture::composer()->update($block, [
+            'set' => ['text' => 'Neu'],
+            'attrs' => ['button.innerContent' => ['desktop' => ['linkUrl' => '/kontakt']]],
+        ], 'changes[0]');
+
+        self::assertStringContainsString('set.text', $result['errors']['changes[0].attrs.button.innerContent']);
+        self::assertSame([], $result['invalid_paths']);
+
+        $separate = ComposeFixture::composer()->update($block, [
+            'set' => ['text' => 'Neu'],
+            'attrs' => ['button.decoration.background' => ['desktop' => ['color' => '#ffffff']]],
+        ], 'changes[0]');
+        self::assertSame([], $separate['errors'], 'different paths may be combined');
+    }
+
     /** Ruling S13: labels go through the choke point; a label that fails is an error, never stored raw. */
     public function testLabelsAreCleanedOrReported(): void
     {