v4.0.0: codec 2.60.4 security release, --pq-box sealed-box mode, F-16 fix

Major release. Highlights:

- Codec: vendored VaptVupt codec moves to canonical 2.60.4 security
  release. Fixes a high-severity OOB heap write in the AVX2 decode fast
  path (reachable on a valid stream sized to exactly content_size, both
  tail variants). Brings CBMC-formally-verified BCJ filters with
  automatic ELF/PE/Mach-O detection. Compressed output stays
  byte-identical (ratio gate Δ 0.00%); wire format unchanged at v1.6.
- New --pq-box sealed-box recipient mode (vendored libpqvaptvupt 0.6.0):
  ML-KEM-768 + X25519 combined via HKDF-SHA256 with domain separation,
  AES-256-CTR + HMAC-SHA256 EtM. Legacy --pq and --pq-sdk stay readable.
- F-16: discloses and fixes a pre-existing data-loss defect in the
  <= 3.8.0 in-tree BCJ encoder. Full back-compat matrix decodes
  byte-exact under 4.0.0; every readable pre-4.0 archive remains readable.

Repository hygiene:
- Sync full 4.0.0 source tree (codec, crypto, SDK, GUI, packaging, tests).
- Remove internal scratch files (PROMPT.md, FORMAL_AUDIT_PROMPT.md)
  and superseded version-specific docs (INTEGRATION_PROTOCOL_2.60.4.md,
  docs/FINDINGS-2.x.md) and a stray test binary.
- Refresh README download/install section to real 4.0.0 release assets;
  bump version badge to 4.0.0.
- Add .gitignore for build outputs (keeps vendored prebuilt libraries).

tests/test_audit_flake.sh

@@ -0,0 +1,85 @@
+#!/bin/bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (c) 2025-2026 Cristian Cezar Moisés
+#
+# Flake-stress harness — §3 of PROMPT.md.
+#
+# Runs every short test suite N times (default 50) and aborts on the
+# first non-deterministic outcome. Specifically targeted at the audit
+# suite, which historically flaked when archive-size variance caused a
+# byte-position-based tamper to land in unauthenticated bytes
+# (finding F-02 in docs/FINDINGS-2.x.md).
+#
+# Usage: bash tests/test_audit_flake.sh [N]
+#
+# Exit codes:
+#   0 — all N runs of every targeted suite passed identically.
+#   1 — at least one run differed (test is flaky). Output names the run.
+
+set -u
+# Default N=20 across 5 suites (~10 min). Pass an arg to override.
+# F-02's repro needed 50 runs to be statistically convincing (~10%
+# baseline flake rate), but at 20 runs we still have ~88% chance of
+# catching a 10%-flake — fine for routine CI. For a hardened audit
+# pass, invoke with 50 or 100 (see PROMPT.md §3).
+N="${1:-20}"
+ZUPT_BIN="${ZUPT_BIN:-./zupt}"
+
+if [ ! -x "$ZUPT_BIN" ]; then
+    echo "  ✗ $ZUPT_BIN not found or not executable. Run 'make' first." >&2
+    exit 1
+fi
+
+run_suite() {
+    local name="$1"; shift
+    local cmd="$*"
+    local pass=0 fail=0 i first_failed_log=""
+    echo ""
+    echo "  ─── $name × $N ───"
+    for i in $(seq 1 "$N"); do
+        local out
+        out=$(bash -c "$cmd" 2>&1)
+        # The convention used by every short suite under tests/ is to
+        # finish with "<N> passed, 0 failed" when the suite is green.
+        if echo "$out" | grep -qE '[0-9]+ passed, 0 failed'; then
+            pass=$((pass+1))
+        else
+            fail=$((fail+1))
+            if [ -z "$first_failed_log" ]; then
+                first_failed_log=$(mktemp)
+                printf '%s\n' "$out" > "$first_failed_log"
+            fi
+        fi
+    done
+    if [ "$fail" -eq 0 ]; then
+        echo "  ✓ $name: $pass/$N green (deterministic)"
+        return 0
+    else
+        echo "  ✗ $name: $pass passed, $fail failed — FLAKY"
+        echo "  First failing run captured at: $first_failed_log"
+        echo "  --- first 30 lines of failing output ---"
+        head -30 "$first_failed_log" | sed 's/^/      /'
+        return 1
+    fi
+}
+
+GLOBAL_FAIL=0
+
+run_suite "tests/test_audit.sh"          "bash tests/test_audit.sh"          || GLOBAL_FAIL=1
+run_suite "tests/test_path_traversal.sh" "bash tests/test_path_traversal.sh" || GLOBAL_FAIL=1
+run_suite "tests/test_arg_order.sh"      "bash tests/test_arg_order.sh"      || GLOBAL_FAIL=1
+run_suite "tests/test_block_swap.sh"     "bash tests/test_block_swap.sh"     || GLOBAL_FAIL=1
+run_suite "tests/test_dedup_props.sh"    "bash tests/test_dedup_props.sh"    || GLOBAL_FAIL=1
+
+echo ""
+if [ "$GLOBAL_FAIL" -eq 0 ]; then
+    echo "  ═══════════════════════════════════════════"
+    echo "  Flake-stress PASS — $N runs × 5 suites all deterministic"
+    echo "  ═══════════════════════════════════════════"
+    exit 0
+else
+    echo "  ═══════════════════════════════════════════"
+    echo "  Flake-stress FAIL — see captured log above"
+    echo "  ═══════════════════════════════════════════"
+    exit 1
+fi