M1: PHPUnit test suite for crypto, repository and resumable runs

Dev tooling via composer (phpunit, psalm, nextcloud/ocp); vendor/ is
mounted from outside the repository by a helper script. 28 unit tests
ported from the phase-0 spikes (round trips, corruption, wrong keys,
dedup, incremental runs, partial restore, tampering, object binding,
time-boxed steps, file changed between steps).

tests/bootstrap.php

@@ -0,0 +1,5 @@
+<?php
+
+declare(strict_types=1);
+
+require __DIR__ . '/../vendor/autoload.php';