test(comms): pin roster presence states, nil-source default, visibility clip (RIG-3537)

[rigel-mintaka]

This PR is part of a stack containing 4 PRs:

1. main
2. test(comms): deepen supervisor orchestration coverage (RIG-3533) #1114
3. test(comms): pin the ring-lag resync arms and replay-boundary exactness (RIG-3538) #1123
4. test(comms): pin dmChannelName properties and DM two-party read scope (RIG-3535) #1136
5. "test(comms): pin roster presence states, nil-source default, visibility clip (RIG-3537)" (this PR)

The roster presence join was asserted by one test covering a single non-default
enum plus the OFFLINE default. Three cases close that.

The presence-state case asserts per-agent (handle -> enum) across WORKING, IDLE,
WAITING and the absent-from-map OFFLINE default in one read. Per-agent is the
point: a set or count assertion still passes when the join maps one agent's enum
onto another's entry, which is the bug this catches. WAITING is the ask-pending
state and the one most likely to regress silently.

The nil-source case pins the hub-less contract documented in presence_source.go —
GetRoster guards on a nil presence source and re-applies the OFFLINE default, so
a caller with no hub wired reads OFFLINE for every agent rather than panicking.

The clipping case covers a member structurally present in the vantage's tree but
invisible to the caller: it must be dropped entirely, not returned blanked. It is
canary-ordered, so an unrelated empty roster cannot pass it.

Mutation-verified, each against production code:

• rotating the presence map by one agent fails the per-agent case
• removing the c.presence != nil guard fails the nil-source case
• disabling the visibility skip fails the clipping case

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-3537

[github-actions]

Compass engineering docs preview: https://compass-comms-rig3537-roster.compass-eng-docs.pages.dev

Deployed from compass-comms/rig3537-roster-presence at ba59e2f.

[trunk-io]

😎 Stack merged successfully - details.