After an internal CAS shard bounce, check_locality() was evaluating
against this_shard_id() of the post-bounce shard — which is the correct
tablet shard — so it returned nullopt, and LWT/SERIAL responses omitted
the tablets-routing-v1 custom payload. The client never learned the
correct tablet map.
Fix by recording the original entry shard in client_state (initialized
to this_shard_id() at construction, preserved across shard bounces via
client_state_for_another_shard) and passing it to check_locality() so
it compares against the client's actual routing decision.
No host_id tracking or forwarded_client_state IDL changes are needed
because CAS shard bounces are always intra-node.
Fixes SCYLLADB-2041
167a3c9c50