quality: add code-style + CGAL-convention checkers (local-only)

Closes the gap "no code-quality / convention gate" from the structural
review.  Three new artefacts, all local-only (CI promotion deferred
until the existing tree is 100 % clean under each):

1. .clang-format — project's existing style mechanically captured
   (4-space indent, opening brace on new line for class/struct/function,
   left-aligned pointer/reference modifiers, aligned `using = ...` blocks,
   100-col loose limit, no include re-ordering — matches code/include/
   today).

2. scripts/quality/clang-format.sh — drift detector.  Dry-run mode by
   default (always exits 0); --strict to fail on drift; --fix to apply
   suggested changes in place.  Skips code/deps/ and macOS-duplicate
   files.

3. scripts/quality/cgal-conventions.py — checker for the CGAL idioms
   that clang-format/clang-tidy cannot express:
     CGAL-1  include-guard format `CGAL_<DIRS>_<FILE>_H`
     CGAL-2  every public header has a `\\file` Doxygen brief
     CGAL-3  no nested namespaces beyond the allowed set
             (CGAL::parameters, CGAL::Conformal_map, internal_np, IO)
     CGAL-4  named-parameter tag types end in `_t`; value object does not
     CGAL-5  no `using namespace ...` at file scope (header leakage)
     CGAL-6  no #define beyond CGAL_* / include-guard

   Result on the current tree: 6 CGAL public headers, 0 violations.
   The checker therefore doubles as documentation of the conventions
   we already follow.

Both are wired into scripts/quality/run-all.sh's fast subset (~5 s
combined wall time).  README.md updated to split the gates into a
"style/convention" group (cheap, run-on-every-commit material) and a
"correctness/quality" group (slow, run-before-tag material).

The reviewer-facing locked-vs-flexible.md gains another "✅ Closed"
row documenting both gates and the 0-violation baseline.

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

scripts/quality/clang-format.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"
+
+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