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/sprite.c

@@ -1,3 +1,8 @@
+/**
+ * @file sprite.c
+ * @brief Implements the sprite subsystem.
+ */
+
 #include <SDL3/SDL.h>
 #include <SDL3_image/SDL_image.h>
 #include <string.h>
@@ -35,6 +40,14 @@ akerr_ErrorContext *akgl_sprite_sheet_coords_for_frame(akgl_Sprite *self, SDL_FR
     SUCCEED_RETURN(e);
 }
 
+/**
+ * @brief Sprite load json spritesheet.
+ * @param json JSON object to parse.
+ * @param sheet Spritesheet used by the sprite.
+ * @param relative_path Base directory for relative asset references.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_* Propagates an error reported by a delegated operation.
+ */
 static akerr_ErrorContext *akgl_sprite_load_json_spritesheet(json_t *json, akgl_SpriteSheet **sheet, char *relative_path)
 {
     PREPARE_ERROR(errctx);