codec: update vendored VaptVupt codec 2.65.0 -> 2.65.3

From vaptvupt-codec tag v2.65.3. Output is byte-identical to 2.65.0 (same
ratio, wire format v1.6 unchanged) but extreme-mode encode is ~1.6-2x faster
(Sprint 132 optimal-parser speedup) and the extreme prepass window allocation
is capped at wlog=20 = 8 MiB virtual instead of up to 128 MiB (Sprint 133
memory hygiene). Our AVX2 offset-read decoder guard is now UPSTREAM (dropped
from the local patch set); the ANS safe-zone 2*SAFEZONE_MAX_RUN reserve is
re-applied on top (still not upstream). make check 16/16, KAT 16/16, cross-
version roundtrip with 5.1.0 archives verified.

vendor/vuptsdk/include/zupt_x25519.h

@@ -0,0 +1,22 @@
+/*
+ * Zupt — Backup-oriented compression with AES-256 encryption
+ * Copyright (c) 2026 Cristian Cezar Moisés
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ *
+ * X25519 Diffie-Hellman key agreement (RFC 7748).
+ * Montgomery ladder — constant-time by construction.
+ */
+#ifndef ZUPT_X25519_H
+#define ZUPT_X25519_H
+
+#include <stdint.h>
+
+/* X25519(scalar, point) → result. All inputs/outputs are 32 bytes.
+ * CT-REQUIRED: Montgomery ladder is inherently constant-time. */
+void zupt_x25519(uint8_t out[32], const uint8_t scalar[32], const uint8_t point[32]);
+
+/* X25519 with the standard basepoint (9).
+ * Used for keygen: public = X25519(private, basepoint). */
+void zupt_x25519_base(uint8_t out[32], const uint8_t scalar[32]);
+
+#endif