Phase 0: repository format with packs, dedup and incremental runs (spike B)

Content-addressed repository on an IBackend (local implementation):
4 MiB blobs, deflate when it helps, each blob encrypted with its id as
associated data, ~32 MiB packs with an encrypted header, one index file
per pack, snapshots as encrypted JSON lines written last.

Spike B: 390 MiB tree, peak 54 MiB; unchanged run reads nothing; a 1 MiB
change in a 300 MiB file uploads one blob; full and partial restore are
bit-identical; tampering is detected.

lib/Repository/PackWriter.php

@@ -0,0 +1,79 @@
+<?php
+
+declare(strict_types=1);
+
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+namespace OCA\NgBackup\Repository;
+
+use OCA\NgBackup\Backend\IBackend;
+use OCA\NgBackup\Crypto\StreamCipher;
+
+/**
+ * Collects encrypted blobs into a pack in memory and uploads it when full.
+ *
+ * Pack layout: blob* | encrypted header | uint32 big-endian header length
+ * The header lists (id, offset, length, rawLength, flags) of every blob, so a pack can be
+ * re-indexed from the pack alone. Each blob is encrypted separately with its id as associated
+ * data, so single blobs can be fetched with a range read and cannot be swapped.
+ *
+ * Only one pack is held at a time (php://temp keeps it in memory up to the limit).
+ */
+final class PackWriter {
+	/** @var resource */
+	private $buffer;
+	private int $size = 0;
+	/** @var list<array{id:string, offset:int, length:int, raw:int, flags:int}> */
+	private array $entries = [];
+
+	public function __construct(
+		private IBackend $backend,
+		private StreamCipher $cipher,
+		private BlobIndex $index,
+		private int $targetSize = 32 * 1048576,
+	) {
+		$this->reset();
+	}
+
+	/** Add a blob that is not yet in the repository. Returns the number of bytes added to the pack. */
+	public function add(string $id, string $encrypted, int $rawLength, int $flags): int {
+		fwrite($this->buffer, $encrypted);
+		$this->entries[] = ['id' => $id, 'offset' => $this->size, 'length' => strlen($encrypted), 'raw' => $rawLength, 'flags' => $flags];
+		$this->size += strlen($encrypted);
+		// Make the blob known immediately, so a later duplicate in the same run is not stored twice.
+		$this->index->addPending($id);
+		if ($this->size >= $this->targetSize) {
+			$this->flush();
+		}
+		return strlen($encrypted);
+	}
+
+	/** Upload the current pack (if any) and record it in the index. */
+	public function flush(): void {
+		if ($this->entries === []) {
+			return;
+		}
+		$packId = bin2hex(random_bytes(16));
+		$header = $this->cipher->encryptString(json_encode($this->entries, JSON_THROW_ON_ERROR), 'pack-header:' . $packId);
+		fwrite($this->buffer, $header . pack('N', strlen($header)));
+		rewind($this->buffer);
+		$path = 'packs/' . substr($packId, 0, 2) . '/' . $packId;
+		$this->backend->put($path, $this->buffer);
+		$this->index->addPack($packId, $this->entries);
+		$this->reset();
+	}
+
+	public function pendingBytes(): int {
+		return $this->size;
+	}
+
+	private function reset(): void {
+		if (isset($this->buffer)) {
+			fclose($this->buffer);
+		}
+		$this->buffer = fopen('php://temp/maxmemory:' . ($this->targetSize + 8 * 1048576), 'w+b');
+		$this->size = 0;
+		$this->entries = [];
+	}
+}