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/ClientLibrariesDesign.md

@@ -63,12 +63,29 @@ Goals:
 
 Non-goals for v1:
 
-- client-side reconnectable sessions,
-- client-side event replay,
+- automatic client-side session reconnection (the library never transparently
+  re-opens a dropped stream on the consumer's behalf — reconnect timing and
+  policy stay an application concern),
+- client-side event replay buffering (the gateway owns the replay ring; the
+  client does not retain its own event history),
 - client-side command batching,
 - synthetic MXAccess events,
 - hiding MXAccess handles behind opaque client-only handles.
 
+The gateway's reconnect-replay *protocol* is, however, consumable from every
+client: each exposes the `after_worker_sequence` resume cursor on
+`StreamEvents` and surfaces the gateway's `ReplayGap` sentinel as a distinct,
+typed, non-terminal signal (CLI-15) so a consumer can detect an evicted-history
+gap and re-snapshot. Per the no-synthesized-events invariant the client never
+fabricates or swallows the sentinel — it only makes the gateway's own signal
+observable. The reaction contract is identical across languages: on a gap,
+discard cached state, re-snapshot, and resume with
+`after_worker_sequence = oldest_available_sequence - 1`. The per-language
+surface follows each idiom (.NET `MxEventStreamItem.IsReplayGap` via
+`StreamEventItemsAsync`; Go `EventResult.ReplayGap`/`IsReplayGap()`; Rust
+`EventItem::ReplayGap`; Python yields a typed `ReplayGap` from
+`stream_events`; Java — pending, tracked with the JDK-17/windows client pass).
+
 ## Public Client Concepts
 
 All languages should expose the same core concepts, using idiomatic naming: