test(e2e): marker-routed multi-turn scripts + observer-scoped fixture clients (RIG-3528)

[rigel-mintaka]

This PR is part of a stack containing 3 PRs:

1. main
2. "test(e2e): marker-routed multi-turn scripts + observer-scoped fixture clients (RIG-3528)" (this PR)
3. test(comms): direct coverage for resolve.go's handle-resolution contracts (RIG-3536) #1030
4. test(e2e): multi-tenant comms visibility transport proof (RIG-3531) #1042

T1 of the RIG-3473 multi-actor comms coverage record: the fixture plumbing the
multi-actor legs (T2-T5, T14-T16) need, plus its own teeth. No leg lands here.

Marker scripts. A marker route settled exactly one TEXT turn, so a marker
could not serve a tool call at all. The naive fix (one CannedTurn per marker)
does not terminate: the route matches a substring of the whole request body and
returns unconditionally, while a tool-call turn needs two POSTs to settle, so
every POST re-serves the tool call and the agent loop spins forever.

So a marker now owns an ordered script with its OWN counter, and its terminal
element repeats once exhausted: [CannedToolCall, CannedText] serves the call,
then the settle, then the settle again. The per-marker counter is deliberately
separate from the positional served (its own mutex) because the invariant that
makes markers useful is that they never consume a positional slot -- that is what
keeps the root-supervisor Setup turn off every leg's script. Extracted
writeCannedTurn so the positional path and the marker routes settle identically;
the tool-call branch was previously inline and reachable only from the positional
path. newCannedMarker keeps its signature and behaviour as the one-element case.

Observer-scoped clients. Every fixture RPC rode the bootstrap-admin bearer,
and an admin sees everything, so no leg could prove a NEGATIVE -- what an account
CANNOT see. AsObserver mints a non-admin bearer over IssueToken and rebuilds both
clients through the existing newAuthedClients dial path. Its teeth are the admin
gate, not a passing read: an adminOnly RPC over the observer client must be
PermissionDenied, since a positive read would also pass if AsObserver silently
handed back the admin token -- which would make every future negative-visibility
assertion vacuous.

Setup wrappers. CreateUser/CreateChannel in CreateAgent's style; T4's first
setup line was unimplementable without them.

private is realized as PLACEMENT, not kind: ChannelKind has no private/public
member (CHANNEL/DM/GROUP_DM), and privacy in this schema is a property of a
channel's group. private=true is ungrouped (owner-scoped, membership-only);
private=false is a fresh top-level SHARED group, so every account can see it.

Verified: the marker teeth fail on the naive implementation ("marker POST#2
carried 1 tool calls, want the text SETTLE"); removing the terminal clamp or
letting the marker branch touch the positional counter each fail too. The two
podman bodies PASS against a real stack (71.9s / 35.3s), not just vet -tags podman. gofmt/vet/vet -tags podman/-race all clean.

Ledger-impact: none

Refs RIG-3528

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

[linear-code]

RIG-3528

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Compass engineering docs preview: https://compass-comms-rig3528-e2e-fi.compass-eng-docs.pages.dev

Deployed from compass-comms/rig3528-e2e-fixture-plumbing at fd0058d.

Changed pages:

• docs/designs/DECISIONS.md