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/Unit/TempDirTrait.php

@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\NgBackup\Tests\Unit;
+
+trait TempDirTrait {
+	/** @var list<string> */
+	private array $tempDirs = [];
+
+	protected function tempDir(): string {
+		$dir = sys_get_temp_dir() . '/ngb-test-' . bin2hex(random_bytes(6));
+		mkdir($dir, 0700, true);
+		$this->tempDirs[] = $dir;
+		return $dir;
+	}
+
+	protected function tearDown(): void {
+		foreach ($this->tempDirs as $dir) {
+			exec('rm -rf ' . escapeshellarg($dir));
+		}
+		$this->tempDirs = [];
+		parent::tearDown();
+	}
+
+	/** @return array<string, string> relative path => sha256 */
+	protected static function hashTree(string $root): array {
+		$out = [];
+		if (!is_dir($root)) {
+			return $out;
+		}
+		$it = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($root, \FilesystemIterator::SKIP_DOTS));
+		foreach ($it as $f) {
+			if ($f->isFile()) {
+				$out[substr($f->getPathname(), strlen($root) + 1)] = hash_file('sha256', $f->getPathname());
+			}
+		}
+		ksort($out);
+		return $out;
+	}
+}