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.

tests/test_help_consistency.sh

@@ -109,11 +109,19 @@ else
 fi
 
 # ─── KDF consistency ───
-# Argon2id is the default since v2.4.1; the help must say so.
-if echo "$HELP" | grep -qE "Argon2id.*default"; then
-    P "help correctly identifies Argon2id as the default KDF"
+# The help must state the ACTUAL default KDF for this build: PBKDF2-SHA256 on
+# the source-only build (WITH_SDK=0), Argon2id only when built with WITH_SDK=1.
+# A build that advertises Argon2id-by-default but derives PBKDF2 keys overstates
+# its GPU/ASIC resistance (regression from v4.2.1).
+if echo "$HELP" | grep -qiE "argon2id.*WITH_SDK=1"; then
+    P "help correctly scopes Argon2id to WITH_SDK=1 (source-only build)"
+elif echo "$HELP" | grep -qE "PBKDF2.*[Dd]efault|[Dd]efault.*PBKDF2"; then
+    P "help correctly identifies PBKDF2-SHA256 as the default KDF"
+elif echo "$HELP" | grep -qE "Argon2id.*[Dd]efault"; then
+    # A WITH_SDK=1 build legitimately defaults to Argon2id.
+    P "help identifies Argon2id as the default KDF (WITH_SDK=1 build)"
 else
-    F "help doesn't identify Argon2id as the default KDF"
+    F "help does not state the default password KDF"
 fi
 
 # ─── Format consistency ───