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/util.c

@@ -1,3 +1,8 @@
+/**
+ * @file util.c
+ * @brief Implements the util subsystem.
+ */
+
 #include <limits.h>
 #include <stdlib.h>
 #include <errno.h>
@@ -15,6 +20,15 @@
 
 #include <akstdlib.h>
 
+/**
+ * @brief Path relative root.
+ * @param root Base directory used to resolve the path.
+ * @param path Path to resolve or transform.
+ * @param dst Output destination populated by the function.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When the corresponding validation or operation fails.
+ * @throws AKERR_OUTOFBOUNDS When the corresponding validation or operation fails.
+ */
 akerr_ErrorContext *akgl_path_relative_root(char *root, char *path, akgl_String *dst)
 {
     PREPARE_ERROR(e);
@@ -80,6 +94,14 @@ akerr_ErrorContext *akgl_path_relative(char *root, char *path, akgl_String *dst)
     SUCCEED_RETURN(e);
 }
 
+/**
+ * @brief Path relative from.
+ * @param path Path to resolve or transform.
+ * @param from Base path from which the result is made relative.
+ * @param dst Output destination populated by the function.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When the corresponding validation or operation fails.
+ */
 akerr_ErrorContext *akgl_path_relative_from(char *path, char *from, akgl_String **dst)
 {
     akgl_String *dirnamestr;