Phase 7.5: Doxygen infrastructure + Phase 8 design freeze

Adds the Doxygen documentation pipeline as the bridge from Phase 7
(porting complete) to Phase 8 (CGAL package). Also captures the
strategic Phase 8 decisions taken on 2026-05-19.

Infrastructure
──────────────
* Doxyfile         — CGAL-style minimal configuration, HTML-only,
                     INPUT=code/include + doc/, excludes deps/ and
                     macOS Finder duplicates
* code/CMakeLists  — `doc` target via find_package(Doxygen QUIET);
                     silently disabled if Doxygen is not installed
* README           — `cmake --build build --target doc` instructions
* .gitignore       — exclude doc/doxygen/ output

Phase 8 strategic decisions (recorded in doc/api/cgal-package.md)
────────────────────────────────────────────────────────────────
* Submission to CGAL: pre-submission-ready, 12+ months horizon, MIT preserved
* Mesh-type flexibility: generic FaceGraph + HalfedgeGraph
* Parameter style: CGAL Named Parameters
* Default kernel: Simple_cartesian<double> (status quo)
* Architecture: 3-layer wrapper, no algorithm duplication
* Acceptance test: Phase 9a (Inversive-Distance) as first new client

CLAUDE.md updated with a compact Phase 8 decision table.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -27,3 +27,7 @@ Testing/
 
 # Claude Code worktrees
 .claude/
+
+# Doxygen output
+doc/doxygen/
+*.dox.tmp