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  and superseded version-specific docs (INTEGRATION_PROTOCOL_2.60.4.md,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_dist_reproducible.sh

@@ -0,0 +1,159 @@
+#!/bin/bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (c) 2025-2026 Cristian Cezar Moisés
+#
+# Sprint 2.4.4 regression test: `make dist` reproducibility.
+#
+# Asserts that running `make dist` twice on the same source tree
+# produces byte-identical tarballs (same sha256, same size). This is
+# the foundational property for downstream Debian / AUR / Homebrew
+# packaging — without it, distros can't pin a sha256 for the source
+# tarball in their recipes.
+#
+# Also asserts that the dist tarball contains the right things:
+#   - source code (src/, include/, tests/)
+#   - the three libzuptsdk symlinks + the real .so file
+#   - no built binaries (zupt, test_vectors, *.o)
+#   - no .git/ tree
+#
+# Exit non-zero on first failure.
+
+set -u
+
+PASS=0
+FAIL=0
+P() { PASS=$((PASS+1)); echo "  ✓ $1"; }
+F() { FAIL=$((FAIL+1)); echo "  ✗ $1"; }
+
+# Run from the project root regardless of where the test was invoked.
+cd "$(dirname "$0")/.."
+
+# 1. First dist build
+make dist >/tmp/dist1.log 2>&1
+RC=$?
+if [ $RC -ne 0 ]; then
+    echo "  ✗ make dist failed on first run; see /tmp/dist1.log"
+    tail -10 /tmp/dist1.log
+    exit 1
+fi
+VERSION=$(grep '^#define ZUPT_VERSION_STRING' include/zupt.h | awk -F'"' '{print $2}')
+# v3.0.0: TARGET=vaptvupt, so the tarball is now /tmp/vaptvupt-${VERSION}.tar.gz.
+# Test both possible filenames so this works on any future rename.
+TARBALL="/tmp/vaptvupt-${VERSION}.tar.gz"
+[ ! -f "$TARBALL" ] && TARBALL="/tmp/zupt-${VERSION}.tar.gz"
+# Derive top-level dir inside the tarball from the filename
+TARBALL_BASE=$(basename "$TARBALL" .tar.gz)   # e.g. vaptvupt-3.0.0
+if [ ! -f "$TARBALL" ]; then
+    echo "  ✗ expected $TARBALL not produced"
+    exit 1
+fi
+P "first make dist produced $TARBALL"
+SHA1=$(sha256sum "$TARBALL" | awk '{print $1}')
+SIZE1=$(wc -c < "$TARBALL")
+cp "$TARBALL" "${TARBALL%.tar.gz}.first.tar.gz"
+
+# 2. Second dist build — should produce byte-identical tarball
+make dist >/tmp/dist2.log 2>&1
+RC=$?
+if [ $RC -ne 0 ]; then
+    echo "  ✗ make dist failed on second run; see /tmp/dist2.log"
+    tail -10 /tmp/dist2.log
+    exit 1
+fi
+SHA2=$(sha256sum "$TARBALL" | awk '{print $1}')
+SIZE2=$(wc -c < "$TARBALL")
+if [ "$SHA1" = "$SHA2" ]; then
+    P "byte-identical sha256 across two runs: $SHA1"
+else
+    F "sha256 diverged: $SHA1 vs $SHA2"
+fi
+if [ "$SIZE1" = "$SIZE2" ]; then
+    P "byte-identical size: $SIZE1"
+else
+    F "size diverged: $SIZE1 vs $SIZE2"
+fi
+
+# 3. Content checks
+NUM_FILES=$(tar tzf "$TARBALL" | wc -l)
+if [ "$NUM_FILES" -gt 100 ]; then
+    P "tarball has $NUM_FILES entries (sanity: > 100)"
+else
+    F "tarball suspiciously small: $NUM_FILES entries"
+fi
+
+if tar tzf "$TARBALL" | grep -q "${TARBALL_BASE}/src/zupt_format.c"; then
+    P "src/zupt_format.c present"
+else
+    F "src/zupt_format.c missing"
+fi
+
+if tar tzf "$TARBALL" | grep -q "${TARBALL_BASE}/include/zupt.h"; then
+    P "include/zupt.h present"
+else
+    F "include/zupt.h missing"
+fi
+
+# All three libzuptsdk variants
+SO_REAL=$(tar tzf "$TARBALL" | grep -c "libzuptsdk.so.2.0.0$")
+SO_LINKS=$(tar tzf "$TARBALL" | grep -cE "libzuptsdk.so$|libzuptsdk.so.2$")
+if [ "$SO_REAL" = "1" ] && [ "$SO_LINKS" = "2" ]; then
+    P "libzuptsdk: 1 real .so + 2 symlinks"
+else
+    F "libzuptsdk shipping wrong: real=$SO_REAL links=$SO_LINKS (expected 1 + 2)"
+fi
+
+# No built binaries (vaptvupt or legacy zupt symlink or test_* harnesses)
+if tar tzf "$TARBALL" | grep -qE "(vaptvupt|zupt)-${VERSION}/(vaptvupt|zupt)(\$|_asan\$)|(vaptvupt|zupt)-${VERSION}/test_vectors\$|(vaptvupt|zupt)-${VERSION}/test_vaptvupt\$"; then
+    F "tarball contains built binaries"
+else
+    P "tarball contains no built binaries"
+fi
+
+# No .o files
+if tar tzf "$TARBALL" | grep -qE "\.o$"; then
+    F "tarball contains stale .o files"
+else
+    P "tarball contains no .o files"
+fi
+
+# No .git
+if tar tzf "$TARBALL" | grep -q "\.git/"; then
+    F "tarball contains .git/ tree"
+else
+    P "tarball contains no .git/ tree"
+fi
+
+# 4. Build & smoke-test from the dist tarball
+WORK=$(mktemp -d)
+( cd "$WORK" && tar xzf "$TARBALL" && cd "${TARBALL_BASE}" && make -j"$(nproc)" >/tmp/distbuild.log 2>&1 ) || {
+    F "build from dist tarball failed; see /tmp/distbuild.log"
+    rm -rf "$WORK"
+    [ "$FAIL" = 0 ] || exit 1
+}
+# v3.0.0: binary may be named `vaptvupt` (default) or legacy `zupt`.
+# Pick whichever the dist-tarball build produced.
+DISTBIN=""
+for cand in vaptvupt zupt; do
+    if [ -x "$WORK/${TARBALL_BASE}/$cand" ]; then DISTBIN="$WORK/${TARBALL_BASE}/$cand"; break; fi
+done
+if [ -n "$DISTBIN" ]; then
+    P "binary builds from dist tarball ($(basename "$DISTBIN"))"
+    "$DISTBIN" version > /tmp/distver.txt 2>&1
+    if grep -q "$VERSION" /tmp/distver.txt; then
+        P "built binary reports correct version ($VERSION)"
+    else
+        F "binary version mismatch: $(cat /tmp/distver.txt)"
+    fi
+else
+    F "no binary produced from dist build"
+fi
+rm -rf "$WORK"
+
+# Cleanup
+rm -f "/tmp/zupt-${VERSION}.first.tar.gz"
+
+echo ""
+echo "  ───────────────────────────────────────"
+echo "  dist reproducibility: $PASS passed, $FAIL failed"
+echo "  ───────────────────────────────────────"
+[ "$FAIL" = 0 ] || exit 1