M2: cursor-based resumable runs with merge-join against the parent

TreeWalker walks one or more roots in a stable segment-wise order with
exclusions and resumes after a cursor; ParentCursor merge-joins the
previous snapshot in the same order. BackupRun no longer keeps a file
list or the parent's entries in memory or in its state.

tests/Unit/Job/TreeWalkerTest.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\NgBackup\Tests\Unit\Job;
+
+use OCA\NgBackup\Job\TreeWalker;
+use OCA\NgBackup\Tests\Unit\TempDirTrait;
+use PHPUnit\Framework\TestCase;
+
+class TreeWalkerTest extends TestCase {
+	use TempDirTrait;
+
+	private function tree(): string {
+		$d = $this->tempDir();
+		foreach (['a.txt', 'a/x', 'a/b/y', 'a-b', 'b', 'skip/me', 'Z'] as $f) {
+			@mkdir(dirname("$d/$f"), 0700, true);
+			file_put_contents("$d/$f", $f);
+		}
+		return $d;
+	}
+
+	public function testOrderMatchesCompare(): void {
+		$paths = array_keys(iterator_to_array((new TreeWalker(['' => $this->tree()]))->walk()));
+		$sorted = $paths;
+		usort($sorted, [TreeWalker::class, 'compare']);
+		$this->assertSame($sorted, $paths);
+		$this->assertSame(['Z', 'a', 'a-b', 'a.txt', 'b', 'skip'], array_values(array_unique(array_map(fn ($p) => explode('/', $p)[0], $paths))));
+	}
+
+	public function testResumeAfterCursorYieldsTheRest(): void {
+		$w = new TreeWalker(['' => $this->tree()]);
+		$all = array_keys(iterator_to_array($w->walk()));
+		foreach ($all as $i => $cursor) {
+			$this->assertSame(array_slice($all, $i + 1), array_keys(iterator_to_array($w->walk($cursor))), "after $cursor");
+		}
+	}
+
+	public function testExcludeAndMultipleRoots(): void {
+		$d = $this->tree();
+		$c = $this->tempDir();
+		file_put_contents("$c/config.php", '<?php');
+		$paths = array_keys(iterator_to_array((new TreeWalker(['config' => $c, 'data' => $d], ['data/skip', 'data/b']))->walk()));
+		$this->assertContains('config/config.php', $paths);
+		$this->assertContains('data/a/b/y', $paths);
+		$this->assertNotContains('data/skip/me', $paths);
+		$this->assertNotContains('data/b', $paths);
+	}
+}