release: restore ZUPT and harden source-only 5.2.2

tests/fixtures/README.md

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+# Compatibility fixtures
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+
+`v5.2.1-encrypted-dedup-disk.zupt.hex` is a textual hexadecimal encoding of
+a 718-byte archive produced by the unmodified VaptVupt `v5.2.1` tag at commit
+`3f897190564d23dd8682f1a07aec62db376b0137`.
+
+The input is four 65,536-byte blocks: `A`, `B`, `B`, then `C`. The archive was
+created with:
+
+```text
+vaptvupt disk backup --dedup -b 65536 \
+  -p vaptvupt-5.2.1-fixture legacy-abbc.zupt legacy-abbc.img
+```
+
+The repeated third block creates a legacy unauthenticated dedup reference to
+the non-zero AAD sequence used by the second DATA frame. The fourth DATA frame
+proves that the 5.2.1 linear AAD sequence advances across that reference. The
+regression decodes the text only in a temporary directory. No binary archive
+is tracked or included as a precompiled program/library.
+
+```text
+input SHA-256:   f144a6486d4971d5af80597dc283254abf3e40a3ea48cb1326eb78a32df009a6
+archive SHA-256: 7aedc693450ff048348730c2d17502499055420d87918d6801dffe87580905bc
+```
+
+The fixture is test data generated by the VaptVupt project and is distributed
+under the project license, AGPL-3.0-or-later.