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>

code/CMakeLists.txt

@@ -140,3 +140,25 @@ install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/include/
 install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/../LICENSE
               ${CMAKE_CURRENT_SOURCE_DIR}/../CITATION.cff
         DESTINATION ${CMAKE_INSTALL_DATADIR}/conformallab)
+
+# ── Doxygen documentation target (Phase 7.5) ──────────────────────────────────
+# Generates HTML API documentation into doc/doxygen/html/.
+# Usage:
+#   cmake --build build --target doc
+#   open doc/doxygen/html/index.html
+#
+# Optional dependency: install Doxygen via `brew install doxygen` (macOS) or
+# `apt install doxygen graphviz` (Linux). The target is silently disabled
+# if Doxygen is not found.
+find_package(Doxygen QUIET)
+if(DOXYGEN_FOUND)
+    set(DOXYGEN_PROJECT_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/..)
+    add_custom_target(doc
+        COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_PROJECT_ROOT}/Doxyfile
+        WORKING_DIRECTORY ${DOXYGEN_PROJECT_ROOT}
+        COMMENT "Generating API documentation with Doxygen"
+        VERBATIM)
+    message(STATUS "Doxygen found: target 'doc' available (cmake --build build --target doc)")
+else()
+    message(STATUS "Doxygen not found — 'doc' target unavailable (install: brew/apt install doxygen)")
+endif()