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>

.gitea/workflows/markdown-links.yml

@@ -0,0 +1,40 @@
+name: Markdown link check
+
+# Verify every internal markdown link in the repo resolves to an existing
+# file (or anchor).  External http(s) links are also probed but with a
+# loose timeout — flaky third-party hosts must not break our CI.
+#
+# Trigger: PRs that touch any *.md file, plus a weekly cron so external
+# link rot is caught even when nobody is editing docs.
+
+on:
+  pull_request:
+    paths:
+      - "**/*.md"
+      - ".gitea/workflows/markdown-links.yml"
+  push:
+    branches:
+      - main
+    paths:
+      - "**/*.md"
+      - ".gitea/workflows/markdown-links.yml"
+  schedule:
+    - cron: "0 5 * * 1"   # Monday 05:00 UTC weekly link-rot check
+  workflow_dispatch: {}
+
+jobs:
+  check:
+    runs-on: eulernest
+    container:
+      image: git.eulernest.eu/conformallab/ci-cpp:latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      # ── Pure-python internal link check (no external network needed) ────
+      # We use the same logic that found the 2 broken links before the
+      # reviewer meeting: parse every [text](path) link, check that the
+      # target file exists relative to the source file's directory.  Skips
+      # http(s)://, mailto:, and pure-anchor (#fragment) links.
+      - name: Internal link check (all *.md files)
+        run: python3 scripts/check-markdown-links.py