## Why
An untrusted exec-server can stop reading requests, never answer them,
or send guessed responses before queued requests are written. Without
client-side admission, the orchestrator can retain unbounded RPC call
futures and request payloads.
We need a hard bound without adding a blanket timeout, because
individual operations already own their timeout and cleanup semantics.
## What changed
- hold one of 64 shared admission permits for the full lifetime of each
regular RPC call, so an early response cannot free capacity while the
call remains queued
- allow `process/terminate` and `fs/close` to use one additional cleanup
permit
- close the transport and fail pending calls if the cleanup permit is
also stuck, allowing teardown or recovery to release remote resources
- leave the wire format and existing timeout behavior unchanged
## Tests
- `rpc_client_call_has_no_implicit_deadline` verifies that ordinary
calls remain untimed
- `rpc_client_bounds_in_flight_calls_and_preserves_cleanup` covers the
regular-call cap, guessed early responses, cleanup admission, and the
cleanup circuit breaker
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