refactor: PostRow::update and PostGuard::assertNoMissedSchedule (M5)

- PostRow::update(id, columns, failure) / ::save(): the slashed wp_update_post of
  internal row writes, refusals as RuntimeException '<failure>: <why>'. Used by
  ContentUpdate::restoreRow, PostCloner::updateRow, MediaMetaWriter apply/restore.
- PostGuard::assertNoMissedSchedule(postId, message) replaces the re-read + check in
  ContentUpdate::restoreRow, PostCloner and DiviSaver::restore (DiviSaver keeps its
  own kses window; M4).
- PostCloner::restore uses Rollback::attempt.
Log messages unchanged.

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

tests/Unit/Integrations/WordPress/PostGuardTest.php

@@ -148,4 +148,20 @@ final class PostGuardTest extends TestCase
         Functions\when('current_user_can')->justReturn(false);
         self::assertSame('lit_forbidden', PostGuard::postType('page', $this->schema(), 'create_posts')->get_error_code());
     }
+
+    public function testAssertNoMissedScheduleRefusesAMissedScheduleWithTheCallersMessage(): void
+    {
+        Functions\when('get_post')->alias(static fn (int $id): ?WP_Post => match ($id) {
+            5 => new WP_Post(['ID' => 5, 'post_status' => 'future', 'post_date_gmt' => '2020-01-01 09:00:00']),
+            6 => new WP_Post(['ID' => 6, 'post_status' => 'publish']),
+            default => null,
+        });
+
+        PostGuard::assertNoMissedSchedule(6, 'nie');
+        PostGuard::assertNoMissedSchedule(7, 'nie');
+
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('Post 5 not written');
+        PostGuard::assertNoMissedSchedule(5, 'Post 5 not written');
+    }
 }