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.

lib/Job/ParentCursor.php

@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Job;
+
+use OCA\NgBackup\Repository\Repository;
+
+/**
+ * Reads the previous snapshot's entries in walk order and answers "what did the parent have
+ * at this path?" for increasing paths (merge-join). Only the current entry is in memory; the
+ * number of entries consumed is part of the resumable state.
+ */
+final class ParentCursor {
+	private ?\Generator $entries = null;
+	private ?array $current = null;
+	private int $consumed = 0;
+
+	public function __construct(Repository $repo, ?string $snapshotId, int $skip = 0) {
+		if ($snapshotId === null) {
+			return;
+		}
+		$this->entries = $repo->entries($snapshotId);
+		$this->current = $this->entries->valid() ? $this->entries->current() : null;
+		while ($this->consumed < $skip && $this->current !== null) {
+			$this->advance();
+		}
+	}
+
+	/** @return array{p:string, s:int, m:int, b:list<string>}|null */
+	public function find(string $path): ?array {
+		while ($this->current !== null) {
+			$c = TreeWalker::compare($this->current['p'], $path);
+			if ($c === 0) {
+				return $this->current;
+			}
+			if ($c > 0) {
+				return null;
+			}
+			$this->advance();
+		}
+		return null;
+	}
+
+	public function position(): int {
+		return $this->consumed;
+	}
+
+	private function advance(): void {
+		$this->entries->next();
+		$this->consumed++;
+		$this->current = $this->entries->valid() ? $this->entries->current() : null;
+	}
+}