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>

src/actor.c

@@ -1,3 +1,8 @@
+/**
+ * @file actor.c
+ * @brief Implements the actor subsystem.
+ */
+
 #include <SDL3/SDL.h>
 #include <SDL3_image/SDL_image.h>
 #include <string.h>
@@ -168,6 +173,15 @@ akerr_ErrorContext *akgl_actor_update(akgl_Actor *obj)
     SUCCEED_RETURN(errctx);
 }
 
+/**
+ * @brief Actor visible.
+ * @param obj Object to initialize, inspect, or modify.
+ * @param camera Camera rectangle used for visibility testing.
+ * @param visible Output set to whether the actor intersects the camera.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_KEY When the corresponding validation or operation fails.
+ * @throws AKERR_NULLPOINTER When the corresponding validation or operation fails.
+ */
 static akerr_ErrorContext *actor_visible(akgl_Actor *obj, SDL_FRect *camera, bool *visible)
 {
     PREPARE_ERROR(errctx);