fix: typed write failures, template rollback, parent and term cleanup

WriteFailed maps to lit_write_failed / lit_partial_write (partial writes are
logged with result partial); old template read before the update; row marker
set before wp_update_post; created terms deleted on rollback; parent must be a
readable non-descendant of a hierarchical type; featured image must be
readable; snapshot keeps empty values; meta comparison after sanitize_meta;
follow-up failures (listener, change log) only add warnings.

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

src/Abilities/AbilityRegistrar.php

@@ -71,6 +71,10 @@ final class AbilityRegistrar
             'execute_callback' => static function (mixed $input = null) use ($ability): mixed {
                 try {
                     return $ability->execute(is_array($input) ? $input : []);
+                } catch (WriteFailed $e) {
+                    $message = sprintf('[wp-lit-mcp] %s failed: %s', $ability->name(), $e);
+                    error_log($message); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_error_log
+                    return $e->toError();
                 } catch (Throwable $e) {
                     $message = sprintf('[wp-lit-mcp] %s failed: %s', $ability->name(), $e);
                     error_log($message); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_error_log