docs+lint: post-merge consistency fixes after PRs #17/#18/#19 landed

Three small cleanups surfaced by running the full gate sweep on the
merged main:

1. CGAL conventions (CGAL-2 \\file briefs)
   doc-only headers `Conformal_map/doxygen_groups.h` and
   `Conformal_map/doxygen_namespaces.h` were missing the `\\file`
   brief required by the CGAL conventions check.  Added both.

2. codespell — three new triggers
   `code/tests/cgal/CMakeLists.txt` uses "honour", `doc/reviewer/hub.html`
   uses `<thead>` (HTML tag, false-positive for "thread"), and
   `doc/roadmap/research-track.md` uses "optimiser".  All three are
   British-English / HTML usage; added to `.codespellrc` ignore list.

3. Doxygen warning in `doc/architecture/compile-time.md:250`
   A trailing backtick-quoted CMake flag at end-of-file confused
   Doxygen's markdown parser into starting a never-closing verbatim
   block.  Rewrote the line to put the prose first and the backtick
   in the middle, not at end-of-file.

All gates green again on the merged main:
  ✅ 259/259 tests pass
  ✅ test-count consistency
  ✅ markdown links 166/166 resolve
  ✅ CGAL conventions 0/8 violations
  ✅ license-headers 68/68 carry MIT SPDX
  ✅ codespell 0 typos
  ✅ shellcheck 0 findings (18 scripts)
  ✅ Doxygen 100% coverage, 0 warnings

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

doc/architecture/compile-time.md

@@ -246,5 +246,5 @@ non-Apple toolchain typically delivers 80 %+ hit rates with ccache.
 The lever is shipped on by default because it's neutral when it
 doesn't help and 10× speedup when it does.
 
-To force-disable for clean from-scratch measurements:
-`-DCONFORMALLAB_USE_CCACHE=OFF`.
+To force-disable for clean from-scratch measurements, pass the
+CMake flag `-DCONFORMALLAB_USE_CCACHE=OFF` at configure time.