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>

doc/api/files/main_8cpp.md

@@ -0,0 +1,13 @@
+# main.cpp
+
+Source: [main.cpp](../../../../src/main.cpp)
+
+## Free Functions
+
+### `int main()`
+
+📍 [main.cpp:3](../../../../src/main.cpp#L3)
+
+---
+
+← [Back to files](index.md) · [Top](../index.md)