[Reactivity] Reconnect, backpressure, guard-gated streams

[cssbruno]

Define reconnect, backpressure, and guard-gated stream behavior for realtime reactivity.

Scope:

• Specify reconnect timing and failure behavior.
• Define backpressure or drop policy.
• Ensure guard/session changes close or deny streams as appropriate.

Acceptance criteria:

• Runtime tests cover reconnect and stream failure behavior.
• Guard denial closes or rejects realtime streams safely.
• Docs explain operational limits and failure modes.