v5.0.0: version bump, audit fixes, documentation overhaul

Version bumped to 5.0.0 across include/zupt.h, all packaging recipes, man
page, and docs.

Audit fixes (pre-5.0.0 review):
- src/zupt_format.c: overflow-safe bound in the solid-mode `test` path
  (off+sz could wrap and drive an OOB read in zupt_xxh64 on a crafted archive;
  the extract path was already hardened, the test path was not).
- gui: run_async now marshals the completion callback onto the GUI thread with
  QueuedConnection (a bare functor connected DirectConnection and touched
  widgets off the worker thread); Extract auto-detect note survives the log
  clear via a new `info` param.
- .github/workflows/ci.yml: trigger on `master` (was main/develop, so CI never
  ran); `make dist` tarball is vaptvupt-*.tar.gz not zupt-*; the ASAN PQ
  round-trip uses native --pq (was --pq-sdk, which fails on the source-only
  build and blocked the release job).

Documentation:
- New AUDIT.md (methodology, FIPS 203 conformance validation, findings, repro).
- CHANGELOG 5.0.0 entry covers the FIPS 203 conformance fix + BREAKING note and
  the GUI/CLI/security/packaging work.
- README "What's new in 5.0.0", download tables (incl. Windows/macOS/BSD +
  portable GUI), version-history row.
- SECURITY.md + THREAT_MODEL.md: ML-KEM-768 documented as FIPS 203, validated
  byte-for-byte against OpenSSL 3.5.
- Accuracy fixes: man page (--kdf default is PBKDF2 on source-only; codec
  2.60.4), rpm %description, debian control/copyright, homebrew header
  (no vendored library on source-only builds).

make check 16/16 (FIPS 203 conformance 3/3, all distro-safe checks).

packaging/debian/control

@@ -25,7 +25,7 @@ Description: Post-quantum backup compression utility (formerly Zupt)
   * Post-quantum hybrid encryption (ML-KEM-768 + X25519, FIPS 203)
   * AES-256-CTR + HMAC-SHA256 authenticated encryption (Encrypt-then-MAC)
   * PBKDF2-SHA256 password key derivation (Argon2id in WITH_SDK=1 builds)
-  * Multi-threaded compression with the VaptVupt LZ + ANS codec 2.48.5
+  * Multi-threaded compression with the VaptVupt LZ + ANS codec 2.60.4
   * Full-disk backup and restore with sparse-region detection
   * End-to-end byte-level tamper detection on encrypted archives
     (0 silent-accept positions in the v1.6 exhaustive byte sweep)