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>

doc/architecture/locked-vs-flexible.md

@@ -266,6 +266,7 @@ mechanical next step rather than a missing piece of theory.
 | **End-to-end smoke (`try_it.sh`) not in CI.** ✅ Closed by `ci/structural-tests`.  Added as a step after the CGAL job. | gate active on every PR | done |
 | **Internal markdown link checker.** ✅ Closed by `ci/structural-tests`.  New `.gitea/workflows/markdown-links.yml` runs on every PR that touches a `*.md` file, plus a weekly cron for external link rot. | gate active on every PR + weekly | done |
 | **Local quality gates (sanitizers, coverage, clang-tidy, multi-compiler, CGAL-version-matrix, reproducible-build, license-headers).** Eight scripts under `scripts/quality/`, driven by `run-all.sh`.  Documented in `scripts/quality/README.md` with promotion-to-CI checklist. | local-only by design; promotion gated on policy text in release-policy.md | done as scripts; 60 SPDX headers missing in `code/include/` will be a follow-up |
+| **Code-style / convention gates (clang-format + CGAL-conventions checker).** ✅ Closed by `ci/structural-tests`. Adds `.clang-format` (project style mechanically captured) + `clang-format.sh` (drift detector, `--fix` mode); `cgal-conventions.py` enforces 6 CGAL-specific idioms (include-guard format, `\file` brief, namespace nesting, named-parameter tag `_t` suffix, no `using namespace`, no stray `#define`).  Both are intentionally local-only — promotion to CI once the existing tree passes `--strict` (today CGAL-conventions does pass: 0/6 violations across 6 CGAL public headers; clang-format drift TBD pending toolchain install). | done as scripts; promotion deferred | done |
 | **CP-Euclidean and Inversive-Distance research-track entries.** Both ship a working DCE solver, but lack the auxiliary utilities the Euclidean / HyperIdeal entries have (curvature inspection helpers, edge-flip Delaunay maintenance for ID).  Out of port scope; listed in [`research-track.md`](../roadmap/research-track.md). | research-track, not blocking | per-utility |
 | **`StereographicUnwrapper`, `CircleDomainUnwrapper`, `CuttingUtility`, `KoebePolyhedron`.** Mentioned in roadmap + research-track docs but not yet ported.  Java versions still authoritative. | documented as Phase 11+ / optional | weeks each — explicit "out of port scope unless requested" |