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/classes/index.md

@@ -0,0 +1,30 @@
+# Classes & Structs
+
+| Name | Brief |
+|---|---|
+| [`Connection`](Connection.md) | — |
+| [`Connections`](Connections.md) | — |
+| [`IdentityTransform`](IdentityTransform.md) | — |
+| [`ImportAltium`](ImportAltium.md) | — |
+| [`ImportBase`](ImportBase.md) | Base class for importing data from a file. |
+| [`ImportMentor`](ImportMentor.md) | — |
+| [`ImportOds`](ImportOds.md) | — |
+| [`Module`](Module.md) | Forward declaration of the [Signals](../classes/Signals.md) class. class. |
+| [`Modules`](Modules.md) | — |
+| [`Net`](Net.md) | — |
+| [`Part`](Part.md) | Forward declaration of the [Module](../classes/Module.md) class. class. |
+| [`Parts`](Parts.md) | — |
+| [`Signal`](Signal.md) | — |
+| [`Signals`](Signals.md) | — |
+| [`System`](System.md) | — |
+| [`SystemElement`](SystemElement.md) | Represents a basic system element with a name. |
+| [`SystemElementContainer`](SystemElementContainer.md) | A container for managing system elements of type T. |
+| [`Transform`](Transform.md) | — |
+| [`TransformRegistry`](TransformRegistry.md) | — |
+| [`Tui`](Tui.md) | — |
+| [`Tui::CommandSpec`](Tui_1_1CommandSpec.md) | — |
+| [`Tui::CommandSpec::Param`](Tui_1_1CommandSpec_1_1Param.md) | — |
+| [`Tui::Prompt`](Tui_1_1Prompt.md) | — |
+| [`VpxTransform`](VpxTransform.md) | — |
+
+← [Top](../index.md)