refactor: documents interfaces

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

lib/Stream/ExpectedTotal.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: Sebastian Krupinski <krupinski01@gmail.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace KTXM\DocumentsManager\Stream;
+
+/**
+ * Implemented by a stream event that declares, up front, how many data frames
+ * are expected to follow. When such an event leads a stream generator, the
+ * envelope writer folds its value into the `control:start` frame's `total`
+ * (the progress denominator) instead of emitting it as a `data` frame.
+ */
+interface ExpectedTotal {
+    public function expectedTotal(): int;
+}