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.

.github/workflows/cross-platform.yml

@@ -0,0 +1,180 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (c) 2025-2026 Cristian Cezar Moisés
+#
+# Cross-platform GUI + CLI binaries, built on REAL Windows and macOS runners.
+#
+# Why a dedicated workflow: the GUI is a PySide6/PyQt6 app and the CLI is
+# portable C11, but self-contained native installers (Windows .exe/.msi,
+# macOS .app/.dmg) can only be produced on the target OS. This workflow builds
+# them on GitHub's windows-latest and macos-latest runners and attaches them to
+# the GitHub release on a `v*` tag. Run it manually with "Run workflow"
+# (workflow_dispatch) to smoke-test the build before tagging.
+#
+# Artifacts produced:
+#   Windows: vaptvupt.exe (CLI, mingw), vaptvupt-gui.exe (PyInstaller onefile),
+#            VaptVupt-Setup-<ver>.exe (Inno Setup installer)
+#   macOS:   vaptvupt (CLI, universal where possible), VaptVupt-<ver>.dmg
+#   All:     vaptvupt-gui-<ver>-portable.zip (Python GUI + launchers)
+
+name: cross-platform
+
+on:
+  push:
+    tags: ['v*']
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  # ─────────────────────────── Windows ───────────────────────────
+  windows:
+    runs-on: windows-latest
+    defaults:
+      run: { shell: msys2 {0} }
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up MSYS2 (mingw gcc + make)
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: UCRT64
+          update: true
+          install: >-
+            mingw-w64-ucrt-x86_64-gcc
+            make
+            coreutils
+      - name: Build CLI (vaptvupt.exe, source-only, C fallback crypto)
+        run: |
+          make CC=gcc WITH_SDK=0 -j2
+          ./vaptvupt.exe version || ./vaptvupt version
+          cp "$(ls vaptvupt.exe vaptvupt 2>/dev/null | head -1)" vaptvupt.exe 2>/dev/null || true
+      - name: Set up Python
+        shell: pwsh
+        run: |
+          # Use the runner's native Python (not MSYS) for PyInstaller so the
+          # produced .exe targets the standard Windows Python ABI.
+          python -m pip install --upgrade pip
+          python -m pip install PySide6 pyinstaller
+      - name: Get version
+        id: ver
+        shell: pwsh
+        run: |
+          $ver = (Select-String -Path include/zupt.h -Pattern '^#define ZUPT_VERSION_STRING "([^"]+)"').Matches.Groups[1].Value
+          "version=$ver" | Out-File -FilePath $env:GITHUB_OUTPUT -Append
+      - name: Bundle GUI with PyInstaller (vaptvupt-gui.exe)
+        shell: pwsh
+        run: |
+          # onefile GUI that carries the CLI beside it via --add-binary.
+          pyinstaller --noconfirm --onefile --windowed `
+            --name vaptvupt-gui `
+            --icon gui/assets/zupt-icon.png `
+            --add-binary "vaptvupt.exe;." `
+            --add-data "gui/assets/zupt-icon.png;assets" `
+            gui/src/zupt_gui.py
+      - name: Build Inno Setup installer
+        shell: pwsh
+        run: |
+          choco install innosetup --no-progress -y
+          & "$env:ChocolateyInstall\bin\ISCC.exe" `
+            "/DAppVersion=${{ steps.ver.outputs.version }}" `
+            packaging/windows/vaptvupt-gui.iss
+      - name: Collect artifacts
+        shell: pwsh
+        run: |
+          $v = "${{ steps.ver.outputs.version }}"
+          New-Item -ItemType Directory -Force out | Out-Null
+          Copy-Item vaptvupt.exe "out/vaptvupt-$v-windows-x86_64.exe"
+          Copy-Item dist/vaptvupt-gui.exe "out/vaptvupt-gui-$v-windows-x86_64.exe"
+          if (Test-Path "packaging/windows/Output") {
+            Copy-Item packaging/windows/Output/*.exe "out/" -ErrorAction SilentlyContinue
+          }
+      - uses: actions/upload-artifact@v4
+        with:
+          name: windows
+          path: out/*
+      - name: Attach to release
+        if: startsWith(github.ref, 'refs/tags/v')
+        uses: softprops/action-gh-release@v2
+        with:
+          files: out/*
+
+  # ─────────────────────────── macOS ───────────────────────────
+  macos:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build CLI (vaptvupt, clang)
+        run: |
+          make CC=clang WITH_SDK=0 -j3
+          ./vaptvupt version
+      - uses: actions/setup-python@v5
+        with: { python-version: '3.12' }
+      - name: Install GUI build deps
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install PySide6 pyinstaller
+          brew install create-dmg || true
+      - name: Get version
+        id: ver
+        run: echo "version=$(awk -F'\"' '/^#define ZUPT_VERSION_STRING/{print $2}' include/zupt.h)" >> "$GITHUB_OUTPUT"
+      - name: Bundle GUI (.app) with PyInstaller
+        run: |
+          pyinstaller --noconfirm --windowed \
+            --name "VaptVupt" \
+            --add-binary "vaptvupt:." \
+            --add-data "gui/assets/zupt-icon.png:assets" \
+            gui/src/zupt_gui.py
+      - name: Build .dmg
+        run: |
+          V="${{ steps.ver.outputs.version }}"
+          create-dmg --volname "VaptVupt $V" --window-size 500 300 \
+            --app-drop-link 350 120 --icon "VaptVupt.app" 150 120 \
+            "VaptVupt-$V.dmg" "dist/VaptVupt.app" || \
+            { mkdir -p dmgroot && cp -R dist/VaptVupt.app dmgroot/ && \
+              hdiutil create -volname "VaptVupt $V" -srcfolder dmgroot -ov -format UDZO "VaptVupt-$V.dmg"; }
+      - name: Collect artifacts
+        run: |
+          V="${{ steps.ver.outputs.version }}"
+          mkdir -p out
+          cp vaptvupt "out/vaptvupt-$V-macos"
+          cp "VaptVupt-$V.dmg" out/
+      - uses: actions/upload-artifact@v4
+        with:
+          name: macos
+          path: out/*
+      - name: Attach to release
+        if: startsWith(github.ref, 'refs/tags/v')
+        uses: softprops/action-gh-release@v2
+        with:
+          files: out/*
+
+  # ─────────────── Portable GUI (works on every OS) ───────────────
+  portable:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Get version
+        id: ver
+        run: echo "version=$(awk -F'\"' '/^#define ZUPT_VERSION_STRING/{print $2}' include/zupt.h)" >> "$GITHUB_OUTPUT"
+      - name: Assemble portable package
+        run: |
+          V="${{ steps.ver.outputs.version }}"
+          D="vaptvupt-gui-$V-portable"
+          mkdir -p "$D/assets"
+          cp gui/src/zupt_gui.py "$D/"
+          cp gui/assets/zupt-icon.png "$D/assets/"
+          cp packaging/portable/vaptvupt-gui.bat "$D/"
+          cp packaging/portable/vaptvupt-gui.command "$D/"
+          cp packaging/portable/vaptvupt-gui.sh "$D/"
+          cp packaging/portable/README.txt "$D/"
+          chmod +x "$D/vaptvupt-gui.command" "$D/vaptvupt-gui.sh"
+          zip -r "$D.zip" "$D"
+      - uses: actions/upload-artifact@v4
+        with:
+          name: portable
+          path: vaptvupt-gui-*-portable.zip
+      - name: Attach to release
+        if: startsWith(github.ref, 'refs/tags/v')
+        uses: softprops/action-gh-release@v2
+        with:
+          files: vaptvupt-gui-*-portable.zip