Document the libakgl API with Doxygen

Add file, structure, global, and function documentation across all libakgl-owned headers and sources, including parameter contracts and likely AKERR/AKGL_ERR exceptions. Add a strict Doxyfile and build the generated API documentation in Gitea CI with warnings treated as failures.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>

.gitea/workflows/ci.yaml

@@ -14,7 +14,7 @@ jobs:
         run: |
           sudo apt-get update -y
           sudo apt-get install -y \
-            cmake gcc pkg-config \
+            cmake doxygen gcc pkg-config \
             libasound2-dev libfreetype-dev libharfbuzz-dev \
             libpng-dev libtiff-dev libwebp-dev \
             libudev-dev libx11-dev libxcursor-dev libxext-dev \
@@ -24,6 +24,8 @@ jobs:
         run: |
           cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
           cmake --build build --parallel
+      - name: Build API documentation
+        run: doxygen Doxyfile
       - name: Test (JUnit)
         run: |
           export LD_LIBRARY_PATH="$PWD/build:$PWD/build/deps/SDL:$PWD/build/deps/SDL_image:$PWD/build/deps/SDL_mixer:$PWD/build/deps/SDL_ttf"