Auto-generated API doc: doxygen → custom Python emitter → doc/api/.

`cmake --build build --target doc` runs Doxygen to produce XML, then
`doc/gen_api_md.py` (~330 lines, stdlib-only) emits a Markdown tree
under `doc/api/` that gitea renders directly in its file browser.

- 24 class/struct pages + 51 source-file pages + indices, with source
  links of the form `../../../../src/...#L42` that gitea turns into
  clickable line-anchored links.
- Doxyfile.in templated by CMake (XML-only output to build/doc/xml/).
- Pure Python emitter, zero external deps — no doxybook2 (not packaged
  on Arch) and no moxygen (avoids Node).
- Target gracefully disabled if Doxygen or Python 3 is missing at
  configure time; regular build target unaffected.

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

DESIGN.md

@@ -164,6 +164,18 @@ Each successful submission appends a single line to the file (so a crash doesn't
 - `System::Load` throws `std::runtime_error("Unknown import type")` for any value outside the three enum cases.
 - `Modules`/`Parts`/etc. have no const-correct iteration on the `*` accessor; iterators on `SystemElementContainer<T>` are available but `begin()`/`end()` are non-const-safe in some places.
 
+## Documentation
+
+`doc/` hosts:
+- `api/` — auto-generated Markdown API reference, browseable directly in gitea.
+- `Doxyfile.in` — CMake-templated Doxygen config (XML-only output to `build/doc/xml/`).
+- `gen_api_md.py` — custom XML → Markdown emitter (~330 lines, stdlib-only Python).
+- `README.md`, `classes.puml` — meta + diagram.
+
+Pipeline: `doxygen` extracts the XML from `src/**`, then `gen_api_md.py` walks `index.xml` plus each `class*.xml` / `*_8hpp.xml` and emits one Markdown file per class/struct/file plus an index. Source-code references use relative paths (`../../../../src/...#L42`) so gitea's renderer turns them into clickable line-anchored links. We picked a custom emitter over `doxybook2` (not in Arch/AUR) and `moxygen` (Node dep) to keep the toolchain pure-C++/Python and trivially modifiable.
+
+Regenerate with `cmake --build build --target doc` after substantive code changes — the target is auto-created when both Doxygen and Python 3 are present at configure time; otherwise the regular build target is unaffected and a status line records which tool is missing. The Markdown output is committed (`doc/api/`), so gitea readers see fresh docs without building.
+
 ## Memory layout for sessions
 
 Persistent notes live in `~/.claude/projects/-home-francois-Projets-essim/memory/`. Index: `MEMORY.md`. Add project/feedback/user/reference entries there when relevant — see top-level Claude Code memory rules.