Replace the per-input `Layer.fresh` + `makeRuntime` facade with a proper
`SessionTransport.Service` exposing a `make(input)` method that returns the
transport handle. The layer is now static; per-input state lives inside the
handle returned by `svc.make(...)`. Update the runtime.ts caller to yield
the service inside `Effect.gen` and wrap the resulting `TransportHandle` in
the existing Promise-shaped `SessionTransport` for the Promise-based
prompt-queue code. `createSessionTransport(...)` is kept as a thin
backward-compatible wrapper for tests.
Kit Langton
248a45801d