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/test_mlkem_fips203.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# FIPS 203 CONFORMANCE test for the in-tree ML-KEM-768.
+#
+# Self-consistency (encaps/decaps round-trip) does NOT prove conformance: a
+# transposed matrix convention round-trips fine but is not interoperable. This
+# test validates against an EXTERNAL FIPS 203 reference — OpenSSL 3.5+, which
+# ships ML-KEM-768 — three ways:
+#   1. deterministic keygen: our ek == OpenSSL's ek for the same seed (d||z)
+#   2. our encaps -> OpenSSL decap: shared secrets match
+#   3. OpenSSL encap -> our decaps: shared secrets match
+#
+# Skips gracefully (exit 0) when the toolchain or an ML-KEM-capable OpenSSL is
+# unavailable, so it is safe inside distro package builds.
+set -u
+echo "ML-KEM-768 FIPS 203 conformance (interop vs OpenSSL)"
+HERE="$(cd "$(dirname "$0")" && pwd)"
+ROOT="$(cd "$HERE/.." && pwd)"
+CC="${CC:-cc}"
+
+command -v openssl >/dev/null 2>&1 || { echo "  - skipped: no openssl"; exit 0; }
+if ! openssl list -kem-algorithms 2>/dev/null | grep -qiE "ML-KEM-768|MLKEM768"; then
+    echo "  - skipped: openssl has no ML-KEM-768 (need 3.5+)"; exit 0
+fi
+command -v "$CC" >/dev/null 2>&1 || CC=gcc
+command -v "$CC" >/dev/null 2>&1 || { echo "  - skipped: no C compiler"; exit 0; }
+command -v od >/dev/null 2>&1 || { echo "  - skipped: no od"; exit 0; }
+
+T=$(mktemp -d); trap 'rm -rf "$T"' EXIT
+H="$T/harness"
+if ! "$CC" -O2 -I"$ROOT/include" -I"$ROOT/src" "$HERE/mlkem_fips203_harness.c" \
+      "$ROOT/src/zupt_mlkem.c" "$ROOT/src/zupt_keccak.c" -o "$H" 2>"$T/cc.err"; then
+    echo "  - skipped: harness build failed"; sed 's/^/    /' "$T/cc.err" | head -3; exit 0
+fi
+hx(){ od -A n -v -t x1 "$1" | tr -d ' \n'; }
+P=0; F=0; ok(){ echo "  ✓ $1"; P=$((P+1)); }; bad(){ echo "  ✗ $1"; F=$((F+1)); }
+cd "$T"
+
+# 1) deterministic keygen ek match
+head -c 64 /dev/urandom > dz.bin
+SEED=$(hx dz.bin)
+openssl genpkey -algorithm ML-KEM-768 -pkeyopt hexseed:"$SEED" -out osl.pem 2>/dev/null
+openssl pkey -in osl.pem -pubout -outform DER -out osl_pub.der 2>/dev/null
+tail -c 1184 osl_pub.der > osl_ek.bin
+MLKEM_RAND="$T/dz.bin" "$H" keygen
+cmp -s ek.bin osl_ek.bin && ok "keygen ek == OpenSSL (byte-for-byte, same seed)" || bad "keygen ek differs from OpenSSL"
+
+# 2) my encaps -> openssl decap
+unset MLKEM_RAND
+"$H" encaps osl_ek.bin >/dev/null 2>&1; cp ss.bin ss_mine.bin
+openssl pkeyutl -decap -inkey osl.pem -in ct.bin -secret ss_osl.bin 2>/dev/null
+cmp -s ss_mine.bin ss_osl.bin && ok "my encaps -> OpenSSL decap: shared secret matches" || bad "my encaps not interoperable"
+
+# 3) openssl encap -> my decap
+HDR=$(( $(stat -c%s osl_pub.der) - 1184 )); head -c "$HDR" osl_pub.der > hdr.bin
+"$H" keygen
+cat hdr.bin ek.bin > my_pub.der
+openssl pkeyutl -encap -pubin -inkey my_pub.der -secret ss_osl2.bin -out ct2.bin 2>/dev/null
+"$H" decaps dk.bin ct2.bin >/dev/null 2>&1; cp ss.bin ss_mine2.bin
+cmp -s ss_mine2.bin ss_osl2.bin && ok "OpenSSL encap -> my decap: shared secret matches" || bad "my decap not interoperable"
+
+echo "  Conformance: $P passed, $F failed"
+[ "$F" -eq 0 ] && exit 0 || exit 1