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.

DISTRIBUTION.md

@@ -10,7 +10,7 @@ Every packaging recipe expects an upstream tarball `vaptvupt-VERSION.tar.gz` pro
 
 ```sh
 make dist
-# → /tmp/vaptvupt-4.1.0.tar.gz
+# → /tmp/vaptvupt-4.2.0.tar.gz
 ```
 
 Re-running `make dist` on the same source tree produces an identical sha256 (verified by `tests/test_dist_reproducible.sh`, wired into `make test`). This lets distros pin a stable hash in their recipes.
@@ -56,13 +56,13 @@ Maintainer flow:
 ```sh
 # 1. Produce the upstream tarball
 make dist
-# → /tmp/vaptvupt-4.1.0.tar.gz
+# → /tmp/vaptvupt-4.2.0.tar.gz
 
 # 2. Upload to a stable URL (e.g. git.securityops.co releases)
 
 # 3. Update packaging/aur/PKGBUILD:
-#    - Set pkgver=4.1.0
-#    - Set sha256sums=("$(sha256sum /tmp/vaptvupt-4.1.0.tar.gz | awk '{print $1}')")
+#    - Set pkgver=4.2.0
+#    - Set sha256sums=("$(sha256sum /tmp/vaptvupt-4.2.0.tar.gz | awk '{print $1}')")
 
 # 4. Generate .SRCINFO
 cd packaging/aur && makepkg --printsrcinfo > .SRCINFO
@@ -73,7 +73,7 @@ makepkg -s
 # 6. Push to AUR
 git clone ssh://aur@aur.archlinux.org/vaptvupt.git aur-vaptvupt
 cp packaging/aur/PKGBUILD packaging/aur/.SRCINFO aur-vaptvupt/
-cd aur-vaptvupt && git add -A && git commit -m "v4.1.0" && git push
+cd aur-vaptvupt && git add -A && git commit -m "v4.2.0" && git push
 ```
 
 User install:
@@ -117,10 +117,10 @@ The `packaging/debian/` tree is a Debian source-package layout. Maintainer flow:
 # 1. Produce the upstream tarball with the standard Debian
 #    orig.tar.gz naming convention:
 make dist
-cp /tmp/vaptvupt-4.1.0.tar.gz /tmp/vaptvupt_4.1.0.orig.tar.gz
+cp /tmp/vaptvupt-4.2.0.tar.gz /tmp/vaptvupt_4.2.0.orig.tar.gz
 
 # 2. Unpack and overlay the debian/ tree:
-cd /tmp && tar xzf vaptvupt_4.1.0.orig.tar.gz && cd vaptvupt-4.1.0
+cd /tmp && tar xzf vaptvupt_4.2.0.orig.tar.gz && cd vaptvupt-4.2.0
 cp -a /path/to/vaptvupt/packaging/debian ./debian
 
 # 3. Build the source package:
@@ -128,7 +128,7 @@ dpkg-buildpackage -S -us -uc       # source-only
 dpkg-buildpackage -b -us -uc       # binary
 
 # 4. Lint:
-lintian vaptvupt_4.1.0-1_*.deb
+lintian vaptvupt_4.2.0-1_*.deb
 
 # 5. Submit via the standard Debian mentors process:
 #    https://mentors.debian.net/intro-maintainers/
@@ -145,7 +145,7 @@ sudo apt install vaptvupt
 ```sh
 # 1. Produce the tarball
 make dist
-cp /tmp/vaptvupt-4.1.0.tar.gz ~/rpmbuild/SOURCES/
+cp /tmp/vaptvupt-4.2.0.tar.gz ~/rpmbuild/SOURCES/
 
 # 2. Drop the .spec into the SPECS directory:
 cp packaging/rpm/vaptvupt.spec ~/rpmbuild/SPECS/
@@ -154,7 +154,7 @@ cp packaging/rpm/vaptvupt.spec ~/rpmbuild/SPECS/
 cd ~/rpmbuild && rpmbuild -ba SPECS/vaptvupt.spec
 
 # 4. Lint:
-rpmlint RPMS/x86_64/vaptvupt-4.1.0-1.fc*.rpm
+rpmlint RPMS/x86_64/vaptvupt-4.2.0-1.fc*.rpm
 
 # 5. Submit via the Fedora new-package review process:
 #    https://docs.fedoraproject.org/en-US/package-maintainers/Package_Review_Process/
@@ -177,7 +177,7 @@ The `packaging/opensuse/` tree carries an RPM `.spec` suited to the Open Build S
 make dist
 
 # 2. In an OBS package checkout (osc), stage the sources and spec:
-cp /tmp/vaptvupt-4.1.0.tar.gz .
+cp /tmp/vaptvupt-4.2.0.tar.gz .
 cp /path/to/vaptvupt/packaging/opensuse/vaptvupt.spec .
 
 # 3. Build locally against a target repository:
@@ -230,7 +230,7 @@ nix build github:cristiancmoises/vaptvupt#vaptvupt
 nix run github:cristiancmoises/vaptvupt#vaptvupt -- version
 
 # 2. To consume from another flake:
-#    inputs.vaptvupt.url = "github:cristiancmoises/vaptvupt?ref=v4.1.0";
+#    inputs.vaptvupt.url = "github:cristiancmoises/vaptvupt?ref=v4.2.0";
 #    packages.x86_64-linux.default = inputs.vaptvupt.packages.x86_64-linux.vaptvupt;
 
 # 3. To submit to nixpkgs (https://github.com/NixOS/nixpkgs):