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/physics.c

@@ -1,3 +1,8 @@
+/**
+ * @file physics.c
+ * @brief Implements the physics subsystem.
+ */
+
 #include <math.h>
 #include <akstdlib.h>
 #include <akgl/physics.h>