rpc_dart_isolate

Isolate-based caller/responder transports for rpc_dart.

• VM: spawns isolates and bridges them with stream-ID multiplexing.
• Web/wasm: uses isolate_manager workers for parity API.
• API: RpcIsolateTransport.spawn returns { transport, kill } pair.

Extracted from rpc_dart_transports to keep web-safe, single-responsibility package.