server-zd: tidy-ups from the #1182 review (swap order, stale gate comments, comment length) #1211

Observed

Small items found in review of #1182 (head 68bbcf3), after it merges. None changes correctness in normal runs.

1. Swap order leaves a tiny window. swap_in_serving_instance in server-zd/crates/dexzd-server/src/server_node/node_lifecycle.rs calls GrpcServer::start_serving (whose task may serve at once) before standby_instance.refuse_public_calls_as_replaced(). A call on a standby connection sent after the serving instance answered its first call could still read the Starting gate state. Fix: call grpc.refuse_public_calls_as_replaced() before the std::mem::replace.
2. Stale doc comments. server-zd/crates/dexzd-server/src/server_node/grpc_server.rs around line 63 ("Public services serve once activated") and line 93 ("answer UNAVAILABLE until activated"): a serving instance now serves from its start, and standby instances never serve. StartupService no longer describes what the gate does; consider a precise name such as PublicCallGate. Update only outdated references; keep the rest of each existing comment.
3. R18 comment length. New comment blocks over 20 words: the doc comment near line 205–208 of server-zd/crates/dexzd-server/tests/serve_process/startup_calls.rs, and the doc comment of NodeGroup::note_serving_period in tests/serve_process/standby_failover/node_group.rs. Shorten them.
4. Hypothesis, unverified: possible flake in standby_failover::step_down::step_down_keeps_the_answer_of_a_call_that_ends_before_the_release. The gate's biased select prefers period_end. If the RPC task ends in the drain but its handler has not returned when the release completes and end_public_calls runs (for example under a host stall), the call becomes UNAVAILABLE and the test fails. Check whether the InvokeRPC handler awaits anything after its RPC task ends; if so, make the test robust or make the handler complete before the release.

Acceptance

• Items 1–3 fixed; make clippy, make rust-rules-check, and cargo test -p dexzd-server --test serve_process startup_calls pass.
• Item 4: either a short note in the PR showing the window does not exist, or a fix with the test run 20 times locally without failure.