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

clients/rust/src/lib.rs

@@ -24,12 +24,14 @@ pub mod version;
 #[doc(inline)]
 pub use auth::{ApiKey, AuthInterceptor};
 #[doc(inline)]
-pub use client::{AlarmFeedStream, EventStream, GatewayClient};
+pub use client::{AlarmFeedStream, EventItem, EventStream, GatewayClient};
 #[doc(inline)]
 pub use error::{CommandError, Error, MxAccessError};
 #[doc(inline)]
 pub use galaxy::{DeployEventStream, GalaxyClient};
 #[doc(inline)]
+pub use generated::mxaccess_gateway::v1::ReplayGap;
+#[doc(inline)]
 pub use options::ClientOptions;
 #[doc(inline)]
 pub use session::{next_correlation_id, Session};