Module handler 

rust__handler__* callbacks invoked by the C++ shim, split by handler-API category. Each submodule holds the callbacks for one section of docs/api/handler.md.

Safety (every callback in these submodules)

• ctx comes from rust__create_engine and has not been destroyed; the C++ shim guards every callback against null on its side, so each Rust callback requires non-null.
• The shim never calls a callback for the same ctx from two threads concurrently, so &mut *ctx is sound inside each callback.
• Pointer/length pairs are valid for the call only; engines must not retain them.