test(comms): deepen supervisor orchestration coverage (RIG-3533)

[rigel-mintaka]

This PR is part of a stack containing 4 PRs:

1. main
2. "test(comms): deepen supervisor orchestration coverage (RIG-3533)" (this PR)
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) #1137

The orchestration file held one case: a supervisor assigning to two
workers. Three contracts around it were unasserted, so a regression in any
of them would have landed green.

Per-topic thread isolation posts four workers' assignments interleaved
across topics rather than grouped, because grouped posts read back
correctly even when topic scoping is broken. Upward reports pin the author
to the worker, not the supervisor or the owner. The membership-scoped audit
asserts an outsider gets exactly zero hits, which is the no-existence-oracle
contract rather than a cosmetic count.

Each case was verified by mutating production and watching it fail: drop
the ListMessages topic filter, credit a different valid channel member as
author, and widen the search membership join past the caller. The audit
case is mutated in the leak direction specifically, since a mutation that
returns nothing exercises the positive assertion instead of the negative.

The record's fourth case, reparent reflected in coordination membership, is
deliberately omitted: TestReparentInEmitsMembershipMove already drives the
identical setup and the same both-halves membership assertions, and also
asserts the removed_account_ids event, so it is strictly stronger.

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

[linear-code]

RIG-3533

[trunk-io]

😎 This pull request was merged.

[github-actions]

Compass engineering docs preview: https://compass-comms-rig3533-orches.compass-eng-docs.pages.dev

Deployed from compass-comms/rig3533-orchestration-depth at f23ccec.

[trunk-io]

This pull request was merged into main as part of stacked PR 1137.