ci+licenses: promote 4 trivial gates to required CI + third-party license doc

Two reviewer-facing additions:

1. New `quality-gates` job in .gitea/workflows/cpp-tests.yml
   ──────────────────────────────────────────────────────────
   Runs in parallel with test-cgal after test-fast.  Installs
   `codespell` + `shellcheck` (apt) into the existing ci-cpp container,
   then executes four scripts strictly (exit 1 on any finding):
     * license-headers.sh   — 66/66 files carry SPDX MIT
     * cgal-conventions.py  — 0 violations across 6 CGAL public headers
     * codespell.sh         — 0 typos across docs + source + scripts
     * shellcheck.sh        — 0 findings across 16 shell scripts

   Each ran at 0 findings locally for weeks before promotion.  The
   gates are now contractual: a regression fails the PR.  Total
   wall-time on the eulernest runner: ~30 s.

2. New code/deps/THIRD-PARTY-LICENSES.md
   ──────────────────────────────────────
   Enumerates every vendored dependency under code/deps/, plus the
   auto-fetched GoogleTest, plus the system-required Boost, with:
     * upstream project + version + SPDX identifier
     * compatibility note for MIT distribution
     * a downstream-packager license matrix (header-only consumer
       vs CLI binary) clarifying the LGPL §3 vs §4 distinction
       relevant to CGAL's header-only consumption

   Required for any future Linux-distribution packaging and for the
   CGAL submission's compliance check.  Cross-referenced from
   doc/architecture/dependencies.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/quality/clang-format 2.sh

@@ -0,0 +1,96 @@
+#!/usr/bin/env bash
+# scripts/quality/clang-format.sh
+#
+# Verify every source file under code/{include,src,tests} matches the
+# project's `.clang-format` policy.  Runs in dry-run mode by default —
+# only reports diffs; never edits files.
+#
+# Pass `--fix` to apply the suggested formatting in place.
+#
+# Local-only.  Promotion to CI is intended once the existing tree is
+# 100 %-conformant; today we report drift but don't fail (the script
+# exits non-zero only with `--strict`).
+#
+# Usage:
+#   bash scripts/quality/clang-format.sh           # dry-run, exit 0 always
+#   bash scripts/quality/clang-format.sh --strict  # dry-run, exit 1 on drift
+#   bash scripts/quality/clang-format.sh --fix     # apply changes
+#
+# Exit codes:
+#   0  no drift, or drift but --strict not set
+#   1  drift detected AND --strict (or fixes applied AND --fix)
+#   2  prerequisite missing
+
+set -uo pipefail
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+cd "$ROOT" || exit 2
+
+command -v clang-format >/dev/null 2>&1 || {
+    echo "FAIL: clang-format not in PATH." >&2
+    echo "        macOS: brew install clang-format" >&2
+    echo "        Linux: sudo apt install clang-format" >&2
+    exit 2
+}
+
+STRICT=0
+FIX=0
+for arg in "$@"; do
+    case "$arg" in
+        --strict) STRICT=1 ;;
+        --fix)    FIX=1 ;;
+        *) echo "Unknown arg: $arg" >&2; exit 2 ;;
+    esac
+done
+
+FILES_LIST="$(find code/include code/src code/tests \
+    \( -name "*.h" -o -name "*.hpp" -o -name "*.cpp" \) \
+    -type f 2>/dev/null \
+    | grep -v "^code/deps/" \
+    | grep -v " 2\." \
+    | sort)"
+
+n_total=0
+n_drift=0
+drift_list=""
+while IFS= read -r f; do
+    [ -z "$f" ] && continue
+    n_total=$((n_total + 1))
+    if [ "$FIX" -eq 1 ]; then
+        clang-format -i "$f"
+    else
+        # --dry-run + -Werror sets non-zero exit when changes would be made.
+        if ! clang-format --dry-run -Werror "$f" >/dev/null 2>&1; then
+            n_drift=$((n_drift + 1))
+            drift_list="$drift_list  $f
+"
+        fi
+    fi
+done <<EOF
+$FILES_LIST
+EOF
+
+echo "clang-format ($(clang-format --version | head -1))"
+echo "Scanned $n_total source files."
+
+if [ "$FIX" -eq 1 ]; then
+    echo "FIX mode: applied formatting in place."
+    exit 0
+fi
+
+if [ "$n_drift" -gt 0 ]; then
+    echo
+    echo "DRIFT: $n_drift file(s) do not match .clang-format policy:"
+    printf "%s" "$drift_list"
+    echo
+    echo "Recovery:"
+    echo "  bash scripts/quality/clang-format.sh --fix   # apply"
+    echo "  git diff                                     # review"
+    if [ "$STRICT" -eq 1 ]; then
+        exit 1
+    fi
+    echo "  (--strict not set → exiting 0 anyway)"
+    exit 0
+fi
+
+echo "OK: every file matches .clang-format policy."
+exit 0