Phase 0: skeleton, streaming encryption (spike A)

Repository skeleton (app id ng_backup, AGPL-3.0-or-later), KeyRing with
KDF subkeys, keyed blob ids and Argon2id key wrapping, and StreamCipher
(secretstream XChaCha20-Poly1305, framed, bounded memory).

Spike A: 1 GiB round trip with memory_limit=64M, peak < 2 MiB, ~160/122
MiB/s; tampering, truncation, wrong key/passphrase are rejected.

appinfo/info.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<info xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://apps.nextcloud.com/schema/apps/info.xsd">
+    <id>ng_backup</id>
+    <name>NG Backup</name>
+    <summary>Encrypted, incremental backups of your Nextcloud with restore from the web interface</summary>
+    <description><![CDATA[Work in progress, not ready for use.]]></description>
+    <version>0.0.1</version>
+    <licence>agpl</licence>
+    <author>Dillard Blom</author>
+    <namespace>NgBackup</namespace>
+    <category>tools</category>
+    <bugs>https://github.com/dillardblom/ng_backup/issues</bugs>
+    <dependencies>
+        <php min-version="8.1"/>
+        <lib>sodium</lib>
+        <nextcloud min-version="32" max-version="35"/>
+    </dependencies>
+</info>