feat: add Jasmin assembly integration for crypto acceleration

- Integrated `zupt_mac_verify_ct` in `zupt_decrypt_buffer()` to replace C XOR loop for HMAC-SHA256
- Integrated `zupt_ct_select_32` in `zupt_mlkem768_decaps()` to replace C `cmov()` for FO transformation
- Added `include/zupt_jasmin.h` with extern declarations and ABI docs
- Added `#ifdef ZUPT_USE_JASMIN` guards with clean C fallbacks in `zupt_crypto.c` and `zupt_mlkem.c`
- Makefile now auto-detects `jasmin/*.s`, assembles and links with `-DZUPT_USE_JASMIN`

Closes #3

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: MIT
+ *
+ * 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