GUI rework for source-only builds + CLI security fixes + packaging + cross-platform scaffolding

Fixes the "GUI functions don't work on GNU Guix" report and a batch of
adversarial-audit findings across the CLI, decoder, crypto and packaging.

GUI (gui/src/zupt_gui.py)
- Root cause of the broken GUI: every encryption path defaulted to the
  libzuptsdk "SDK v2" modes, which are absent from the source-only build and
  exit 1. Reworked Keys/Compress/Extract around the native modes with a
  build-aware PQ-mode selector: Hybrid (--pq, default), Full PQ (--pq-only),
  and SDK v2 only when the binary reports WITH_SDK support. Capability is
  detected from `version` ("Build:"/"KDF:" lines) with a `help` fallback.
- Extract/Verify gain a PQ private-key input with auto-detect (reads the
  archive via `info` to pick --pq vs --pq-only). Verify could not verify any
  PQ archive before (password field only).
- run_async now holds a LIST of in-flight (thread, worker) refs; DiskTab's two
  buttons previously shared one slot, so a second op GC'd the first QThread
  mid-run. About tab corrected (codec 2.60.4, PBKDF2 default, --pq-only, URL).

CLI (src/zupt_main.c)
- compress -p <archive> <files> swallowed the archive name as the password and
  truncated the first input file (silent data loss, exit 0). Added a
  data-loss guard: refuse to overwrite an existing non-.zupt file as the output
  archive unless -y/--force; plus a self-overwrite guard.
- compress <archive> <src> -p <pw> wrote an UNENCRYPTED archive (exit 0)
  because options after the first positional were treated as files. Now errors
  on a misplaced option (with a `--` escape for real dashed filenames).
- Removed duplicated dead --pq-box/--pq-sdk branches. version/banner/usage now
  state the build's real KDF (PBKDF2 on source-only) and repo URL; examples
  lead with native --pq / --pq-only.

Security (audit findings)
- vv_decoder.c: the two AVX2 fast-path token decoders read a 2-/3-byte match
  offset after only checking 1 byte remained -> heap over-read on a crafted
  archive. Added the `ip + off_bytes > ip_end` guard the tail path already had.
- zupt_crypto.c: wipe ML-KEM/X25519 secret-key buffers when hybrid decrypt
  init fails on key read (matches the pq-only path).
- zupt_format.c: bound attacker-controlled encryption_header_off before the
  (off_t)+7 arithmetic in the info enc_type reader (avoid signed-overflow UB).

Packaging (would fail source-only)
- debian/rules (staged into debian/zupt, installed vendored .so), aur/PKGBUILD,
  nix/flake.nix, homebrew (vendored .so + nonexistent AUDIT.md, stale /zupt
  URLs) now build source-only. opensuse spec %files ships the shell completions
  make install writes (rpmbuild no longer fails on unpackaged files);
  _service + debian/control point at the vaptvupt repo; KDF claim corrected.

Cross-platform GUI packaging (new)
- packaging/portable/: OS-agnostic GUI package (zupt_gui.py + .bat/.command/.sh
  launchers + README) that runs on Windows/macOS/Linux/BSD with Python+PySide6.
- .github/workflows/cross-platform.yml: builds real native binaries on
  windows-latest + macos-latest runners (CLI, PyInstaller GUI, Inno Setup
  installer, .dmg) and the portable zip, attaching them to the release on tag.
- packaging/windows/vaptvupt-gui.iss: Inno Setup installer script.

Validation: make check 16/16 (all distro-safe checks), GUI imports + command
contracts verified against the fixed CLI. test_help_consistency updated to
assert the truthful (build-aware) default KDF.

packaging/homebrew/vaptvupt.rb

@@ -22,8 +22,8 @@
 
 class Vaptvupt < Formula
   desc "Post-quantum backup compression utility (ML-KEM-768 + AES-256-CTR + HMAC-SHA256)"
-  homepage "https://git.securityops.co/cristiancmoises/zupt"
-  url "https://git.securityops.co/cristiancmoises/zupt/releases/download/v4.2.1/vaptvupt-4.2.1.tar.gz"
+  homepage "https://git.securityops.co/cristiancmoises/vaptvupt"
+  url "https://git.securityops.co/cristiancmoises/vaptvupt/releases/download/v4.2.1/vaptvupt-4.2.1.tar.gz"
   version "4.2.1"
   sha256 "REPLACE_WITH_SHA256_OF_RELEASE_TARBALL"
   license "AGPL-3.0-or-later"
@@ -31,30 +31,16 @@ class Vaptvupt < Formula
   depends_on "python@3.12" => :test  # only for test-suite tamper harness
 
   def install
-    # macOS build: no Jasmin, C-fallback crypto paths are used.
-    # The Makefile auto-detects Jasmin availability and falls back cleanly.
+    # Source-only build (WITH_SDK=0): native crypto only, no vendored libraries.
+    # macOS uses the C-fallback crypto paths (no Jasmin); the Makefile detects
+    # this and falls back cleanly.
     ENV["CFLAGS"] = "#{ENV.cflags} -O2 -std=c11 -Wall -Wextra"
 
-    system "make", "-j#{ENV.make_jobs}"
-    system "make", "DESTDIR=#{prefix}", "PREFIX=", "install"
+    system "make", "WITH_SDK=0", "-j#{ENV.make_jobs}"
+    system "make", "DESTDIR=#{prefix}", "PREFIX=", "WITH_SDK=0", "install"
 
-    # Vendored libzuptsdk goes into lib/zupt/ with @loader_path rpath.
-    # Note: Linux ships .so.2.0.0; macOS .dylib equivalent must be built
-    # separately by the vendored makefile. For the initial Homebrew
-    # submission this assumes the upstream tarball includes a .dylib build;
-    # if not, build it here.
-    lib_zupt = lib/"zupt"
-    lib_zupt.mkpath
-    if File.exist?("vendor/zuptsdk/libzuptsdk.dylib")
-      cp "vendor/zuptsdk/libzuptsdk.dylib", lib_zupt
-    elsif File.exist?("vendor/zuptsdk/libzuptsdk.so.2.0.0")
-      # Fallback: link Linux-style .so on macOS (works for direct loads but
-      # not for dlopen-on-Darwin scenarios). Upstream is tracking this.
-      cp "vendor/zuptsdk/libzuptsdk.so.2.0.0", lib_zupt
-    end
-
-    # Docs
-    doc.install "README.md", "SECURITY.md", "CHANGELOG.md", "AUDIT.md"
+    # Docs (no vendored .so/.dylib in the source-only build).
+    doc.install "README.md", "SECURITY.md", "CHANGELOG.md"
   end
 
   test do