feat(clients): CLI-15 surface ReplayGap reconnect sentinel as typed signal (4/5 clients)

The gateway emits a ReplayGap sentinel MxEvent at the head of a StreamEvents
stream resumed via after_worker_sequence when the requested cursor predates the
oldest retained event. Clients previously ignored it, silently mis-treating a
lossy resume as continuous. Each client now surfaces the sentinel as a distinct,
typed, non-terminal signal (never synthesized, never swallowed) so a consumer can
detect the gap and re-snapshot; resume contract is
after_worker_sequence = oldest_available_sequence - 1.

- .NET: MxEventStreamItem (IsReplayGap/ReplayGap/Event) via new StreamEventItemsAsync
  + AsStreamItemsAsync extension. Build clean, 87 passed.
- Go: EventResult.ReplayGap field + IsReplayGap(); ReplayGap type alias. build/vet/test clean.
- Rust: EventItem enum (Event/ReplayGap); EventStream now yields Result<EventItem, Error>;
  CLI renders REPLAY_GAP line / replayGap JSON. fmt/check/test/clippy clean.
- Python: ReplayGap dataclass; stream_events yields pb.MxEvent | ReplayGap. 131 passed.
- Shared docs: ClientLibrariesDesign non-goals reframed (reconnect-replay protocol is
  consumable; auto-reconnect stays a non-goal); CrossLanguageSmokeMatrix resume-gap note.

Java client is deferred to the windev batch (no local JRE); CLI-15 stays open until it lands.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

docs/CrossLanguageSmokeMatrix.md

@@ -30,6 +30,14 @@ Each client entry defines commands for the same required operation sequence:
 The optional `write` command is documented separately because writing changes
 provider state and should only run when the operator supplies a safe test value.
 
+When `stream-events` is resumed with an `after_worker_sequence` cursor that
+predates the oldest event still in the gateway's replay ring, the gateway emits a
+single `ReplayGap` sentinel at the head of the stream. Every client surfaces this
+as a distinct, typed, non-terminal signal (see each client README); the resume
+contract is `after_worker_sequence = oldest_available_sequence - 1`. The default
+smoke sequence opens a fresh stream (no cursor) and does not exercise the gap
+path; a resume-with-gap fixture case is tracked separately (TST-24).
+
 ## Integration Gate
 
 Cross-language smoke execution is opt-in. Runners should skip the matrix unless