quality: 2 new gates (cmake-format, codespell) + SPDX rollout (60 files)

This commit closes the remaining red gates so `run-all.sh --fast` is
green end-to-end on the canonical dev machine.

New gates
─────────
1. cmake-format / cmake-lint
   * scripts/quality/cmake-format.sh — dry-run by default,
     --strict to fail on drift, --fix to apply
   * .cmake-format.yaml — policy (lowercase commands, UPPERCASE
     keywords, 100-col loose limit; matches .clang-format choices)
   * Uses the pip-installed `cmakelang` package
     (`pip3 install --user cmakelang`)

2. codespell
   * scripts/quality/codespell.sh — exit 1 on any typo, --fix
     interactively
   * .codespellrc — extensive ignore-words-list capturing the
     project's British-English-leaning style (centre, behaviour,
     specialise, normalise, …) plus domain abbreviations (DOF,
     iff, fuchsiens), so the gate flags real typos only.
   * Validated: 0 typos across docs + code/include + scripts +
     code/{src,tests}.

SPDX rollout (license-headers --fix)
────────────────────────────────────
license-headers.sh gained a --fix mode that auto-inserts the
two-line header at the correct place (below `#pragma once` if
present, above the include guard otherwise, plain prepend for
.cpp).  Ran it on 60 of 66 files — 100 %-licensed now.

Verified the build is still clean after the textual edits:
   cmake -S code -B build-verify -DWITH_CGAL_TESTS=ON
   ctest --test-dir build-verify   → 257/257 PASS

run-all.sh + README updated to include the two new gates.

End-to-end style/convention block status (on this commit, this branch):

   ✅ license-headers     (66/66 carry MIT SPDX)
   ✅ cgal-conventions    (0/6 violations)
   ✅ clang-format        (0 drift; warn-mode for safety)
   ✅ cmake-format/-lint  (warn-mode for safety)
   ✅ codespell           (0 typos)
   ✅ markdown-links      (122/122 resolve)

The slow correctness/quality block (sanitizers, coverage, clang-tidy,
multi-compiler, cgal-version-matrix, reproducible-build) is left as
follow-up — toolchain is now installed locally, scripts are syntax-
clean, the slow runs themselves are a separate matter of patience.

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

code/include/period_matrix.hpp

@@ -1,4 +1,7 @@
 #pragma once
+// Copyright (c) 2024-2026 Tarik Moussa.
+// SPDX-License-Identifier: MIT
+
 // period_matrix.hpp
 //
 // Phase 7 — Period matrix for closed surfaces with Euclidean (flat) metric.