ci+quality: structural gates (CI: 3 new; local: 7 new + .clang-tidy)

CI gates (active on every PR via .gitea/workflows/)
───────────────────────────────────────────────────
1. test-count consistency
   cpp-tests.yml gains a step after test-cgal that runs
   `scripts/check-test-counts.sh` against the just-built ./build dir
   (reuse via new BUILD_DIR env var, ~5 s overhead).  Drift between
   `doc/api/tests.md` and ctest reality now fails the PR.

2. End-to-end smoke
   `scripts/try_it.sh` (the documented user quick-start) is now part of
   the CGAL job, so README quick-start regressions fail the PR rather
   than silently breaking when users land.

3. Internal markdown link checker
   New `.gitea/workflows/markdown-links.yml` + `scripts/check-markdown
   -links.py`.  PRs that touch any *.md file run the check; main pushes
   trigger it too; a weekly cron catches external link rot.  Pure
   Python, no third-party action.  Validated against the current tree:
   122 internal links across 37 *.md files, 0 broken.

Local quality scripts (`scripts/quality/`, not in CI)
─────────────────────────────────────────────────────
* `license-headers.sh`   — `SPDX-License-Identifier: MIT` audit over
                            code/{include,src,tests}/.  Currently
                            reports 60/66 files missing it — that's
                            a follow-up; the script captures the
                            structural gap.
* `sanitizers.sh`        — ASan + UBSan over the fast test suite.
* `coverage.sh`          — gcov/lcov line + branch coverage of
                            code/include/, HTML report under
                            build-coverage/lcov-html/.
* `clang-tidy.sh`        — runs the curated `.clang-tidy` policy over
                            every public header.
* `multi-compiler.sh`    — sequential build + test against every
                            detected g++/clang++ (auto-discovery or
                            explicit list).
* `cgal-version-matrix.sh`— sequential build + CGAL test suite against
                            every CGAL tree under `~/cgal/<ver>/` (or
                            via `CGAL_ROOTS=...` env var).
* `reproducible-build.sh`— two `Release -j1` builds, fail if any test
                            executable byte-differs.
* `run-all.sh`           — driver: `--fast` for the ~5-min subset,
                            no arg for the ~25–40 min full sweep;
                            captures per-gate logs to
                            build-quality-logs/.

+ `.clang-tidy`          — curated, deliberately-small policy (only
                            checks that fire on OUR code, never on
                            transitive CGAL/Eigen/Boost headers).

+ `scripts/quality/README.md` — explains the structure, lists each
                            gate's wall-time + prereqs, and codifies
                            the promotion path: a gate moves into CI
                            only when it's green on the dev machine
                            AND has a recovery-instructions paragraph
                            in `doc/release-policy.md`.

Doc updates
───────────
`doc/architecture/locked-vs-flexible.md` (reviewer-facing) gains 4
"closed" rows in the limitations table — the 3 CI gates above and the
local quality-script suite.

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

scripts/check-test-counts.sh

@@ -31,16 +31,27 @@ command -v cmake >/dev/null || { echo "FAIL: cmake not in PATH" >&2; exit 2; }
 command -v ctest >/dev/null || { echo "FAIL: ctest not in PATH" >&2; exit 2; }
 
 # ── Build ───────────────────────────────────────────────────────────────────
-# Re-use existing build-cgal/ if it exists with the right flags; otherwise
-# create a throwaway build-counts/ directory.
-BUILD_DIR=""
-if [ -f build-cgal/CMakeCache.txt ] && grep -q "WITH_CGAL_TESTS:.*=ON\|WITH_CGAL:.*=ON" build-cgal/CMakeCache.txt; then
+# Priority order:
+#   1. `BUILD_DIR` env var (CI sets it to the dir cpp-tests.yml just built).
+#   2. existing build-cgal/ if it has WITH_CGAL_TESTS=ON.
+#   3. throwaway build-counts/ — full build from scratch.
+if [ -n "${BUILD_DIR:-}" ]; then
+    if [ ! -f "$BUILD_DIR/CMakeCache.txt" ]; then
+        echo "FAIL: BUILD_DIR=$BUILD_DIR has no CMakeCache.txt" >&2
+        exit 2
+    fi
+    if ! grep -q "WITH_CGAL_TESTS:.*=ON\|WITH_CGAL:.*=ON" "$BUILD_DIR/CMakeCache.txt"; then
+        echo "FAIL: BUILD_DIR=$BUILD_DIR was not configured with WITH_CGAL_TESTS=ON" >&2
+        exit 2
+    fi
+elif [ -f build-cgal/CMakeCache.txt ] && grep -q "WITH_CGAL_TESTS:.*=ON\|WITH_CGAL:.*=ON" build-cgal/CMakeCache.txt; then
     BUILD_DIR=build-cgal
+    cmake --build "$BUILD_DIR" -j"$(nproc 2>/dev/null || sysctl -n hw.logicalcpu 2>/dev/null || echo 2)" >/dev/null
 else
     BUILD_DIR=build-counts
     cmake -S code -B "$BUILD_DIR" -DWITH_CGAL_TESTS=ON -DCMAKE_BUILD_TYPE=Release >/dev/null
+    cmake --build "$BUILD_DIR" -j"$(nproc 2>/dev/null || sysctl -n hw.logicalcpu 2>/dev/null || echo 2)" >/dev/null
 fi
-cmake --build "$BUILD_DIR" -j"$(nproc 2>/dev/null || sysctl -n hw.logicalcpu 2>/dev/null || echo 2)" >/dev/null
 
 # ── Get actual counts ───────────────────────────────────────────────────────
 cd "$BUILD_DIR"