feat(go): add diagnostic event subscription to client

[explise]

Description

Summary

The Go SDK currently has no way for callers to observe client-level lifecycle events (connection state changes, auth state changes). The Rust SDK exposes these via Client::subscribe_events() returning a stream of DiagnosticEvent. This issue ports that capability to the Go SDK.

Motivation

The Go TCP client already has TODO stubs in place for event emission, indicating this was anticipated:

• foreign/go/client/tcp/tcp_core.go:399 — // TODO publish event
• foreign/go/client/tcp/tcp_core.go:461 — // TODO event pushing logic
• foreign/go/client/tcp/tcp_core.go:475 — `// TODO push shutdown event.

This is groundwork for adding a high-level IggyConsumer abstraction to the Go SDK (planned as a follow-up). Diagnostic events let the consumer detect disconnects, re-join consumer groups eagerly on reconnect, and resume cleanly. Beyond the consumer use case, the events are also useful for application-level observability (logging connection drops, exposing health metrics, etc.).

Scope

In scope:

1. Add a DiagnosticEvent type in foreign/go/contracts/ mirroring the Rust enum (5 variants, with a String() method matching the snake_case form used in Rust).
2. Add SubscribeEvents() <-chan DiagnosticEvent to the Client interface in foreign/go/contracts/client.go.
3. Implement a small fan-out broadcaster in the TCP client so multiple subscribers can each receive every event independently.
4. Wire emission into the TCP client:
   • Connected — on initial connect and on every successful auto-reconnect (existing reconnect loop at foreign/go/client/tcp/tcp_core.go:86-100, 351-401)
   • Disconnected — at the existing TODO at tcp_core.go:399
   • Shutdown — at the existing TODO at tcp_core.go:475
   • SignedIn — after successful LoginUser
   • SignedOut — after successful LogoutUser
5. Integration test that subscribes, exercises connect → disconnect → reconnect → login → logout → close, and asserts the full event sequence is delivered.

Component

Go SDK

Proposed solution

No response

Alternatives considered

No response

[explise]

@hubcio @chengxilo please assign this to me

[chengxilo]

I'm sorry I don't have that privilege, but I believe @hubcio can help u.