Write a consumer contract guide for WebSocket and MQTT delivery, ordering, reconnects, and duplicates

[jillesvangurp]

Summary

Recent docs now describe queue drops, deduplication, and some transport behavior, but issue #26 still needs a single downstream-consumer contract for stream consumers.

Scope

• document delivery semantics for outbound WebSocket and MQTT event streams in one consumer-facing guide
• define reconnect expectations and how consumers should handle duplicates or replayed observations
• clarify ordering assumptions, slow-consumer/backpressure consequences, and consumer responsibilities
• align the guide with current implementation limits rather than idealized future behavior

Why

This extracts the still-missing documentation slice from issue #26 after the recent runtime and connector docs updates.

Part of #26
Related to #31
Related to #32
Related to #33
Related to #34
Related to #35