feat: content-create and content-update with schema validation

All-or-nothing writes: validation (fields, terms, featured image, parent,
template) before any write; failed extras delete the new draft (create) or
restore revision, old columns, fields, terms and featured image (update).
content-update requires rev; Rev::ofPost covers content, title, excerpt,
status and post_modified_gmt. MetaFieldSource::write() now throws on failure.

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

tests/Stubs/WpPost.php

@@ -21,6 +21,7 @@ if (! class_exists('WP_Post')) {
         public $post_date = '2026-09-25 10:00:00';
         public $post_date_gmt = '2026-09-25 08:00:00';
         public $post_modified = '2026-09-25 10:00:00';
+        public $post_modified_gmt = '2026-09-25 08:00:00';
         public $post_mime_type = '';
         public $comment_status = 'closed';
         public $ping_status = 'closed';