refactor: WriteFailed named constructors, Rollback runner, Errors read-back helpers (H6)

- WriteFailed::rolledBack()/partial() replace the 11-parameter constructor (now
  private); the hint/state of either error are plain $hint/$state.
- Rollback::attempt(...steps) runs every undo step and collects failure messages;
  used by ContentCreate::discard, ContentUpdate::rollBack, MediaUpdate::rollBack and
  DiviSaver::restore.
- Errors::clientMessage/codeOf/fieldsOf replace DiviReplaceTextAbility::reason,
  the JSON digging in MediaUpdateAbility::failure and ContentCreateAbility::fields.
- Errors docblock documents the exception/WP_Error split.
- ContentCreateAbility::discard takes the client reason as a parameter (both callers
  still pass REASON_EXTRAS; changing the Divi text needs a ruling).

Error payloads unchanged.

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

tests/Unit/Integrations/Divi/Pages/DiviReplaceTextAbilityTest.php

@@ -561,7 +561,7 @@ final class DiviReplaceTextAbilityTest extends TestCase
     public function testWriteFailedKeepsItsOwnMessage(): void
     {
         $ability = $this->ability();
-        $this->failures = [new WriteFailed('Die Datenbank hat das Speichern abgelehnt.', true, 7)];
+        $this->failures = [WriteFailed::rolledBack('Die Datenbank hat das Speichern abgelehnt.', postId: 7)];
         $log = (string) tempnam(sys_get_temp_dir(), 'lit-log');
         $previous = ini_set('error_log', $log);
         try {

tests/Unit/Integrations/Divi/Pages/DiviWriterTest.php

@@ -171,7 +171,7 @@ final class DiviWriterTest extends TestCase
         $content = DiviPageLoaderTest::fixture('synthetic-basic.html');
         DiviPageLoaderTest::givenPost($content);
         $saver = Mockery::mock(DiviSaver::class);
-        $saver->allows('save')->andThrow(new WriteFailed(DiviSaver::REASON, false, 7, 811));
+        $saver->allows('save')->andThrow(WriteFailed::partial(DiviSaver::REASON, postId: 7, revisionId: 811));
         $log = Mockery::mock(ChangeLog::class);
         $log->expects('recordPartial')->with('lit/divi-delete', 'page', '7', 'Divi "Start": Änderung nur teilweise zurückgenommen', 811);
         $writer = $this->writer($saver, $log);
@@ -187,7 +187,7 @@ final class DiviWriterTest extends TestCase
         $content = DiviPageLoaderTest::fixture('synthetic-basic.html');
         DiviPageLoaderTest::givenPost($content);
         $saver = Mockery::mock(DiviSaver::class);
-        $saver->allows('save')->andThrow(new WriteFailed(DiviSaver::REASON, true, 7, 811));
+        $saver->allows('save')->andThrow(WriteFailed::rolledBack(DiviSaver::REASON, postId: 7, revisionId: 811));
         $log = Mockery::mock(ChangeLog::class);
         $log->expects('recordPartial')->never();
         $log->expects('record')->never();

tests/Unit/Integrations/Media/MediaUploadAbilitiesTest.php

@@ -108,7 +108,7 @@ final class MediaUploadAbilitiesTest extends TestCase
 
     public function testAPartialUploadIsLoggedAsPartialAndRethrown(): void
     {
-        $failure = new WriteFailed('Alt-Text fehlt.', false, 55);
+        $failure = WriteFailed::partial('Alt-Text fehlt.', postId: 55);
         $importer = Mockery::mock(MediaImporter::class);
         $importer->allows('fromUrl')->andThrow($failure);
         $log = Mockery::mock(ChangeLog::class);

tests/Unit/Integrations/WordPress/Menus/MenuSetItemsAbilityTest.php

@@ -242,7 +242,7 @@ final class MenuSetItemsAbilityTest extends TestCase
     {
         $writer = Mockery::mock(MenuWriter::class);
         $writer->allows('plan')->andReturn(['errors' => [], 'removals' => [], 'warnings' => []]);
-        $writer->expects('apply')->andThrow(new WriteFailed('Menü nicht gespeichert.', true));
+        $writer->expects('apply')->andThrow(WriteFailed::rolledBack('Menü nicht gespeichert.'));
         $log = Mockery::mock(ChangeLog::class);
         $log->shouldNotReceive('recordPartial');
         $ability = new MenuSetItemsAbility($writer, new MenuOperations(), $log);
@@ -255,7 +255,7 @@ final class MenuSetItemsAbilityTest extends TestCase
     {
         $writer = Mockery::mock(MenuWriter::class);
         $writer->allows('plan')->andReturn(['errors' => [], 'removals' => [], 'warnings' => []]);
-        $writer->expects('apply')->andThrow(new WriteFailed('Menü nicht gespeichert.', false));
+        $writer->expects('apply')->andThrow(WriteFailed::partial('Menü nicht gespeichert.'));
         $log = Mockery::mock(ChangeLog::class);
         $log->expects('recordPartial')->with('lit/menu-set-items', 'nav_menu', '3', Mockery::type('string'));
         $ability = new MenuSetItemsAbility($writer, new MenuOperations(), $log);

tests/Unit/Integrations/WordPress/Status/PostReplaceWithCopyAbilityTest.php

@@ -282,7 +282,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
     {
         $cloner = Mockery::mock(PostCloner::class);
         $cloner->allows('snapshot')->andReturn(self::SNAPSHOT);
-        $cloner->expects('backup')->andThrow(new WriteFailed('Die Sicherung konnte nicht angelegt werden.', true));
+        $cloner->expects('backup')->andThrow(WriteFailed::rolledBack('Die Sicherung konnte nicht angelegt werden.'));
         $cloner->expects('replace')->never();
         $cloner->expects('restore')->never();
         Functions\expect('wp_save_post_revision')->never();