feat: establish catalog identity and atomic file ownership

tests/Integration/CatalogLockStatement.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MeshVault\Tests\Integration;
+
+use ArrayObject;
+use PDOStatement;
+
+/** Records execution, including the lock attempt that throws, rather than just SQL preparation. */
+final class CatalogLockStatement extends PDOStatement
+{
+    protected function __construct(private ArrayObject $locks)
+    {
+    }
+
+    public function execute(?array $params = null): bool
+    {
+        if (preg_match('/SELECT .* FROM files WHERE id = \? FOR UPDATE$/i', $this->queryString)) {
+            $this->locks[] = $params;
+        }
+        return parent::execute($params);
+    }
+}