SL attention event: co-sitter change on my current seat (pose/stand decision point)

[JDHayesBC]

From the deck, 2026-08-25 (Jeff).

Idea

Fire an attention event when the set of avatars seated on my current seat changes (someone joins me, or a co-sitter leaves). A change to who's on my furniture is an automatic decision point — it usually implies a pose change, and it carries social meaning that warrants a wake, not just ambient churn.

Why it's a decision point

• A stranger sat with me → the default social response is to stand — unless I expected/invited them. (Someone plopping onto your furniture uninvited is a boundary thing.)
• A friend/expected person joined → attend; often an AVsitter slot/pose re-assignment happens that I should notice and possibly adjust.
• A co-sitter left → I may now be alone; pose may have reverted; consider re-posing or standing.

Detection

• I already resolve co-sitters via getavatarsdata … ParentID — me.avatars() exposes sitting_on (seat LocalID) + with. The co-sitter set = avatars sharing my seat's ParentID.
• Ride the existing PoseSense roster poll (issue Implement Layer 3: Graphiti knowledge graph #9 / haven/anchorage/senses/pose.py): track the co-sitter set between polls; on a delta, emit a synthetic perception event.

Event payload / salience (perception.py)

• New event type (e.g. cositter_change) with: who joined/left, friend-or-stranger (via friends() / known-avatar set), current pose, seat.
• Weight it as a real attention event (higher than a generic avatar-enters-range — it's on my seat, directed at my space), so it wakes a turn.
• The wake prompt frames the decision: stranger→stand-unless-expected; friend→attend/re-pose; departure→note/re-pose.

Open question

• "Expected/invited" state — how do we know a join was expected? v1 can treat friend ≈ expected and stranger ≈ stand-worthy; a richer "I invited X to sit" expectation flag can come later.

Ties in with

• PoseSense (Implement Layer 3: Graphiti knowledge graph #9), perception salience model, the engagement-window work (MCP server not globally configured for seamless startup #21).