v4.2.0: full (pure) post-quantum mode + critical dedup nonce fix

Add a native full post-quantum encryption mode and fix a critical
keystream-reuse bug in deduplicated encrypted archives.

Full post-quantum mode (--pq-only)
- New envelope type 0x06 (ZUPT_ENC_PQ_ONLY): ML-KEM-768 (FIPS 203) as
  the sole key-establishment mechanism, with no classical X25519
  component. Archive key = SHA3-512(ml_ss || ml_ct || "ZUPT-PQ-ONLY-v1").
- For compliance postures that require a single NIST-standardised PQ
  primitive with no classical KEM in the envelope (CNSA 2.0-style
  "PQ-only"). Hybrid --pq stays the recommended default; --pq-only has
  no classical fallback, so a break of ML-KEM-768 alone breaks it.
- keygen --pq-only / keygen --pub --pq-only (ZPQK magic, 1200B pub /
  3600B priv; not interchangeable with hybrid --pq keys). Wrong or
  tampered ciphertext is rejected via ML-KEM FO implicit rejection plus
  the HMAC-SHA256 Encrypt-then-MAC envelope. In-tree, default build.

Security (critical): AES-256-CTR keystream reuse under --dedup
- Dedup assigns block sequence 0 to every data block (the sentinel that
  keeps cross-file dedup references authenticating consistently). The
  per-block nonce was base_nonce XOR block_seq, so under --dedup every
  block collapsed to the same nonce, reusing the CTR keystream across
  distinct plaintexts (a many-time-pad). Each block now uses a fresh
  random 128-bit nonce stored in the block prefix and bound into the
  block MAC; block_seq is still bound as MAC AAD. Regression test:
  tests/test_dedup_nonce.sh. Re-encrypt any --dedup encrypted archives
  written by <= 4.1.0.

Other
- keygen --sdk / --box on a source-only build now fails with a clear
  message pointing to native --pq / --pq-only (or a WITH_SDK=1 build).
- Documentation: README, SECURITY, THREAT_MODEL, man page, CHANGELOG,
  and all packaging recipes updated for the new mode and the security
  fix; version bumped to 4.2.0. Wire format v1.6 unchanged (0x06 is
  additive).

Validation: make check 16/16, quick suite 11/11 (incl. PQ-only),
dedup-nonce regression (all block nonces distinct), cppcheck clean.

tests/run_quick.sh

@@ -23,10 +23,14 @@ E=$(find "$T/o7" -name a.txt -type f 2>/dev/null|head -1); [ -n "$E" ] && diff -
 $Z keygen -o "$T/k.key" 2>/dev/null && $Z keygen --pub -o "$T/p.key" -k "$T/k.key" 2>/dev/null
 $Z compress --pq "$T/p.key" "$T/8.zupt" "$T/d/" 2>/dev/null && $Z extract --pq "$T/k.key" -o "$T/o8" "$T/8.zupt" 2>/dev/null
 E=$(find "$T/o8" -name a.txt -type f 2>/dev/null|head -1); [ -n "$E" ] && diff -q "$T/d/a.txt" "$E" >/dev/null 2>&1 && ok "PQ" || fl "PQ"
+# Full post-quantum (ML-KEM-768 only, no X25519) round-trip.
+$Z keygen --pq-only -o "$T/kq.key" 2>/dev/null && $Z keygen --pub --pq-only -o "$T/pq.key" -k "$T/kq.key" 2>/dev/null
+$Z compress --pq-only "$T/pq.key" "$T/9.zupt" "$T/d/" 2>/dev/null && $Z extract --pq-only "$T/kq.key" -o "$T/o9" "$T/9.zupt" 2>/dev/null
+E=$(find "$T/o9" -name a.txt -type f 2>/dev/null|head -1); [ -n "$E" ] && diff -q "$T/d/a.txt" "$E" >/dev/null 2>&1 && ok "PQ-only" || fl "PQ-only"
 R=$($Z test "$T/1.zupt" 2>&1); echo "$R"|grep -q "0 failed" && ok "Integrity" || fl "Integrity"
 # F-01 (2.2.4): every help command verb starts its own line.
 # Pre-fix output ran "keygen … Key generation  zupt version" on one
 # wrapped line due to a missing \n in src/zupt_main.c:41.
 HC=$($Z help 2>&1 | grep -cE '^  (vaptvupt|zupt) ')
 [ "$HC" -ge 10 ] && ok "Help command lines ($HC)" || fl "Help command lines ($HC, need ≥10)"
-echo ""; echo "  Results: $P passed, $F failed (10 tests)"; [ "$F" -eq 0 ] && exit 0 || exit 1
+echo ""; echo "  Results: $P passed, $F failed (11 tests)"; [ "$F" -eq 0 ] && exit 0 || exit 1