crypto: make ML-KEM-768 genuinely FIPS 203-conformant (validated vs OpenSSL)

The in-tree ML-KEM-768 was round-3 CRYSTALS-Kyber mislabelled "FIPS 203" and
was NOT interoperable with a compliant ML-KEM. Discovered and fixed by
validating against OpenSSL 3.5's FIPS 203 ML-KEM-768 as an oracle.

Three deviations, all fixed in src/zupt_mlkem.c:
1. Matrix  transpose convention. FIPS 203 K-PKE.KeyGen samples
   Â[i][j] = SampleNTT(XOF(ρ, j, i)) and K-PKE.Encrypt uses (ρ, i, j); the
   code had both index orders swapped. It was self-consistent (encaps/decaps
   round-tripped) but transposed vs the standard — which is exactly why a
   self-consistency-only round-trip test never caught it. With the same seed,
   keygen now produces a byte-identical ek to OpenSSL.
2. Shared-secret KDF. FIPS 203 returns K = G(m‖H(ek))[0:32] directly; removed
   the round-3 final K = KDF(K̄‖H(c)) step (encaps + decaps success key).
3. Implicit rejection. Now K̄ = J(z‖c) = SHAKE256(z ‖ full-ciphertext) instead
   of the round-3 KDF(z‖H(c)).

Validation (tests/test_mlkem_fips203.sh + mlkem_fips203_harness.c, wired into
make check): against OpenSSL 3.5 ML-KEM-768 —
  - deterministic keygen (same d‖z seed) -> byte-identical ek
  - our encaps -> OpenSSL decap: shared secret matches
  - OpenSSL encap -> our decaps: shared secret matches
The harness feeds a fixed random stream (MLKEM_RAND) so the FIPS 203 seed is
reproducible; the test skips gracefully without an ML-KEM-capable openssl.

BREAKING: --pq / --pq-only keys and archives from <= 4.2.1 no longer decrypt
(the KEM math changed). Regenerate keys and re-encrypt. Password mode and plain
compression are unaffected; wire format stays v1.6. make check 16/16.

tests/mlkem_fips203_harness.c

@@ -0,0 +1,65 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Deterministic ML-KEM-768 harness for FIPS 203 conformance testing against an
+ * external reference (OpenSSL 3.5+). Uses the project's PUBLIC KEM API over raw
+ * FIPS 203 byte strings (ek=1184, dk=2400, ct=1088, ss=32).
+ *
+ *   keygen              -> ek.bin, dk.bin   (d,z consumed from MLKEM_RAND if set)
+ *   encaps <ek.bin>     -> ct.bin, ss.bin   (m consumed from MLKEM_RAND if set)
+ *   decaps <dk.bin> <ct.bin> -> ss.bin
+ *
+ * When env MLKEM_RAND names a file, zupt_random_bytes() consumes it SEQUENTIALLY
+ * (keygen reads d then z; encaps reads m), so the same FIPS 203 seed fed to a
+ * reference implementation produces byte-identical ek/dk/ct/ss.
+ *
+ * Built by tests/test_mlkem_fips203.sh against src/zupt_mlkem.c + src/zupt_keccak.c
+ * (no -DZUPT_USE_JASMIN, so the portable constant-time select is used). */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include "zupt_mlkem.h"
+
+static FILE *g_rand; static int g_rand_init;
+void zupt_random_bytes(uint8_t *buf, size_t len) {
+    if (!g_rand_init) {
+        const char *p = getenv("MLKEM_RAND");
+        g_rand = fopen(p ? p : "/dev/urandom", "rb");
+        g_rand_init = 1;
+    }
+    if (!g_rand || fread(buf, 1, len, g_rand) != len) { fprintf(stderr, "rand fail\n"); exit(2); }
+}
+int zupt_ct_memeq(const void *a, const void *b, size_t n) {
+    const uint8_t *x = a, *y = b; uint8_t d = 0;
+    for (size_t i = 0; i < n; i++) d |= (uint8_t)(x[i] ^ y[i]);
+    return d == 0 ? 1 : 0;
+}
+static void wr(const char *p, const uint8_t *b, size_t n) {
+    FILE *f = fopen(p, "wb");
+    if (!f || fwrite(b, 1, n, f) != n) { fprintf(stderr, "write %s\n", p); exit(2); } fclose(f);
+}
+static size_t rd(const char *p, uint8_t *b, size_t n) {
+    FILE *f = fopen(p, "rb"); if (!f) { fprintf(stderr, "open %s\n", p); exit(2); }
+    size_t g = fread(b, 1, n, f); fclose(f); return g;
+}
+int main(int argc, char **argv) {
+    if (argc >= 2 && !strcmp(argv[1], "keygen")) {
+        uint8_t ek[1184], dk[2400];
+        if (zupt_mlkem768_keygen(ek, dk)) return 2;
+        wr("ek.bin", ek, 1184); wr("dk.bin", dk, 2400); return 0;
+    }
+    if (argc == 3 && !strcmp(argv[1], "encaps")) {
+        uint8_t ek[1184], ct[1088], ss[32];
+        if (rd(argv[2], ek, 1184) != 1184) return 2;
+        if (zupt_mlkem768_encaps(ct, ss, ek)) return 2;
+        wr("ct.bin", ct, 1088); wr("ss.bin", ss, 32); return 0;
+    }
+    if (argc == 4 && !strcmp(argv[1], "decaps")) {
+        uint8_t dk[2400], ct[1088], ss[32];
+        if (rd(argv[2], dk, 2400) != 2400) return 2;
+        if (rd(argv[3], ct, 1088) != 1088) return 2;
+        if (zupt_mlkem768_decaps(ss, ct, dk)) return 2;
+        wr("ss.bin", ss, 32); return 0;
+    }
+    fprintf(stderr, "usage: keygen | encaps <ek> | decaps <dk> <ct>\n");
+    return 1;
+}