feat: added host-uri frame and rpc bridge for read/write/cancel routing
- Added `set_host_uri_schemes` and host-uri frame/type definitions; documented read/write/cancel behavior.
- Added `RpcHostUriBridge` in rpc mode to register schemes, dispatch read/write/cancel ops, and clear pending requests.
- Added internal URL write support with lowercased scheme matching, handler routing, and hashline-prefixed success output.
- Added Python host-uri APIs/exports, cancellable client request handling, and host-uri read/write test coverage.
can1357
bddf9989b5