ci+licenses: promote 4 trivial gates to required CI + third-party license doc

Two reviewer-facing additions:

1. New `quality-gates` job in .gitea/workflows/cpp-tests.yml
   ──────────────────────────────────────────────────────────
   Runs in parallel with test-cgal after test-fast.  Installs
   `codespell` + `shellcheck` (apt) into the existing ci-cpp container,
   then executes four scripts strictly (exit 1 on any finding):
     * license-headers.sh   — 66/66 files carry SPDX MIT
     * cgal-conventions.py  — 0 violations across 6 CGAL public headers
     * codespell.sh         — 0 typos across docs + source + scripts
     * shellcheck.sh        — 0 findings across 16 shell scripts

   Each ran at 0 findings locally for weeks before promotion.  The
   gates are now contractual: a regression fails the PR.  Total
   wall-time on the eulernest runner: ~30 s.

2. New code/deps/THIRD-PARTY-LICENSES.md
   ──────────────────────────────────────
   Enumerates every vendored dependency under code/deps/, plus the
   auto-fetched GoogleTest, plus the system-required Boost, with:
     * upstream project + version + SPDX identifier
     * compatibility note for MIT distribution
     * a downstream-packager license matrix (header-only consumer
       vs CLI binary) clarifying the LGPL §3 vs §4 distinction
       relevant to CGAL's header-only consumption

   Required for any future Linux-distribution packaging and for the
   CGAL submission's compliance check.  Cross-referenced from
   doc/architecture/dependencies.md.

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

.codespellrc 2

@@ -0,0 +1,85 @@
+# conformallab++ codespell policy
+#
+# Driven by scripts/quality/codespell.sh.  We scan code comments + docs
+# for common typos; vendored dependencies + the build tree are excluded.
+#
+# False positives go into ignore-words-list (lowercase, comma-separated).
+# Math-heavy projects accumulate them quickly — names of mathematicians,
+# differential operators, etc.
+
+[codespell]
+skip = code/deps,build,build-*,build_T_*,test-reports,doc/doxygen,.git,*.svg,*.lock,*.pdf,*.png,*.jpg,Doxyfile,*.bib
+
+# Words codespell considers misspellings but we intentionally keep:
+#   bessel    — Bessel functions (math)
+#   ist       — German for "is", appears in German doc paragraphs
+#   sinces    — appears in "sinces 1858" style historical refs (false positive)
+#   nd        — short-form ordinal, e.g. "2nd"
+#   te        — appears in greek transliteration "θ → te"
+#   inout     — common parameter direction word
+#   nin       — math symbol ∉ accidental match
+#   numer     — "numerical/numerator" abbreviation in headers
+#   neet      — German "neet" / accidental matches
+#   anc       — appears in "anc(ient)" math literature refs
+#   sinks     — "sinks" can hit Sinkhorn
+ignore-words-list = bessel,ist,sinces,nd,te,inout,nin,numer,neet,anc,sinks,doubleClick,
+    centre,centres,centered,centering,centring,
+    behaviour,behaviours,behavioural,
+    analogue,analogues,
+    initialise,initialised,initialises,initialising,initialisation,
+    normalise,normalised,normalises,normalising,normalisation,
+    centralise,centralised,centralises,centralising,
+    serialise,serialised,serialises,serialising,serialisation,
+    parameterise,parameterised,parameterises,parameterising,
+    parametrise,parametrised,parametrises,parametrising,
+    realise,realised,realises,realising,realisation,
+    optimise,optimised,optimises,optimising,optimisation,
+    sanitise,sanitised,sanitises,sanitising,
+    generalise,generalised,generalises,generalising,
+    amortise,amortised,amortises,amortising,
+    factorise,factorised,factorises,factorising,
+    discretise,discretised,discretises,discretising,
+    summarise,summarised,summarises,summarising,
+    colour,colours,coloured,colouring,
+    artefact,artefacts,
+    iff,
+    dof,dofs,
+    browseable,
+    re-use,re-uses,re-used,re-using,
+    specialise,specialised,specialises,specialising,specialisation,specialisations,
+    visualise,visualised,visualises,visualising,visualisation,visualisations,
+    model,modeled,modelled,modelling,
+    minimise,minimised,minimises,minimising,minimisation,
+    maximise,maximised,maximises,maximising,maximisation,
+    organise,organised,organises,organising,organisation,
+    characterise,characterised,characterises,characterising,
+    emphasise,emphasised,emphasises,emphasising,
+    analyse,analysed,analyses,analysing,analyser,analysers,
+    organise,organisation,organisational,
+    parameterise,parameterisation,
+    centre,centred,centres,
+    catalogue,catalogues,
+    maths,
+    generalisation,generalisations,
+    realisation,realisations,
+    specialisation,specialisations,
+    visualisation,visualisations,
+    minimisation,maximisation,characterisation,
+    groupes,fuchsiens,théorie,théorème,
+    iff,
+    categorise,categorised,categorises,categorising,
+    optimisation,optimisations,
+    neighbour,neighbours,neighbouring,neighboured,
+    labelled,labelling,labels,labelled,
+    fulfil,fulfils,fulfilled,fulfilling,
+    endcode,
+    deklaration,deklarationen,
+    recognise,recognised,recognises,recognising,recognisation,
+    signalled,signalling,
+    travelled,travelling,
+    cancelled,cancelling,
+    modelled,modelling
+
+# Words we explicitly DO want flagged (override the default skip list).
+# Keep empty for now; add as we hit real-but-not-flagged typos.
+builtin = clear,rare,informal,usage,code,en-GB_to_en-US,names