tarantool-etcd-kbc Conformance test: gRPC Watch on a follower replica Lined Up

tests/conformance/replicaset_smoke_test.go covers ReadReplicatesToFollowers (Range on each follower after a leader Put) but nothing exercises a Watch stream *pinned to a follower endpoint* while mutations happen on the leader. The Lua integration test (test/grpc_integration_test.lua:547) explicitly punts: "True replicaset behaviors (watch deliveries across nodes ...) need a real Raft cluster and live in the Go harness."

Watch on a follower has its own moving parts that Range does not:
- the watcher fiber lives on the follower
- kv_history rows arrive via replication (relay → applier), not via the local mutation
- the box.on_commit broadcast must fire on the applier path (see memory feedback_tarantool_trigger_broadcast — broadcast from on_commit, not on_replace, so the woken reader sees committed state on the follower side too)

Without this test, a regression in the follower-side watch wake-up would slip through the rs3 suite.

- Reuse mustDial(t, []string{followerEndpoint}) from existing rs3 tests — same dial pattern as ReadReplicatesToFollowers.
- Use a short context.WithTimeout (5 s wall, 2 s wait-for-event) — replication lag on the local rs3 harness is sub-ms.
- Use harness.UniqueKey(t) for key isolation.
- Skip on non-rs3 backend (the single-node tarantool backend has no follower).
- Mirror the synchronous "wait for created chunk" pattern jsonclient/watch already uses, even on the gRPC client — avoids the race where the Put commits before the Watch is registered.

1. New test in tests/conformance/replicaset_smoke_test.go that:
   a. Spawns the rs3 backend, identifies leader + followers via rs.LeaderEndpoint / rs.Endpoints.
   b. Opens a Watch stream pinned to each follower endpoint (one subtest per follower).
   c. Issues a Put on the leader.
   d. Asserts the follower-side watcher receives the PUT event within a short deadline (≤2 s on a quiet box).
   e. Asserts the event header.revision matches the leader-side Put response revision.
2. Additional subtest: range watch ("/rs_smoke/watch/", "\0") on a follower receives multiple events in revision order when the leader issues several Puts.
3. Optional subtest: prev_kv populated correctly on the follower-side event when WithPrevKV is requested.
4. Suite stays green with CONFORMANCE_BACKEND=tarantool_rs3 go test -count=1 -run TestReplicasetSmoke -v.

Source: gap surfaced while porting docs/TODO.md to bd on 2026-05-19. Cross-refs: tests/conformance/replicaset_smoke_test.go ReadReplicatesToFollowers (line ~160), test/grpc_integration_test.lua:547 comment about which behaviors live where, memory feedback_tarantool_trigger_broadcast.