M8a — v1.1: heartbeat + ack

[nficano]

Goal

v1.1: heartbeat + ack.

Scope

• Add session.ping/session.pong/session.ack payloads (_messages/session.py extends)
• Add feature flag heartbeat and ack to V1_1_FEATURES; wire intersection helper into _runtime/session.py + _client/client.py
• Add heartbeat ticker (out-of-TaskGroup per architecture §2)
• Add back_pressure status emission
• Examples: heartbeat/, ack_backpressure/

Spec §

§6.2, §6.4, §6.5

Risk

M (heartbeat-coupling risk)