Skip the response body when presence state hasn't changed

[josephfusco]

Return a cheap "nothing changed" signal when a room's state hash matches what the client already has, instead of rebuilding and shipping the full JSON payload every tick. Standard conditional-request pattern (ETag-equivalent), applied to the Heartbeat response shape rather than a full HTTP 304, given Heartbeat's single-request-many-features design.

Part of #267.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Assisting with investigation and drafting

[i-am-chitti]

@josephfusco I'm picking this up.