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/character.c

@@ -1,3 +1,8 @@
+/**
+ * @file character.c
+ * @brief Implements the character subsystem.
+ */
+
 #include <SDL3/SDL.h>
 #include <SDL3_image/SDL_image.h>
 #include <akerror.h>
@@ -85,6 +90,14 @@ void akgl_character_state_sprites_iterate(void *userdata, SDL_PropertiesID regis
     } FINISH_NORETURN(errctx);
 }
 
+/**
+ * @brief Character load json state int from strings.
+ * @param states JSON array of actor-state names.
+ * @param dest Output destination populated by the function.
+ * @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 *akgl_character_load_json_state_int_from_strings(json_t *states, int *dest)
 {
     int i = 0;
@@ -109,6 +122,13 @@ static akerr_ErrorContext *akgl_character_load_json_state_int_from_strings(json_
     SUCCEED_RETURN(errctx);
 }
 
+/**
+ * @brief Character load json inner.
+ * @param json JSON object to parse.
+ * @param obj Object to initialize, inspect, or modify.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When the corresponding validation or operation fails.
+ */
 static akerr_ErrorContext *akgl_character_load_json_inner(json_t *json, akgl_Character *obj)
 {
     PREPARE_ERROR(errctx);