refactor(web): one predicate decides what anchors into a root's cluster

[gbasin]

Small invariant cleanup in the same area as #562. No reachable behavior change.

The gap

Two conditions in Timeline.tsx described the same set — agent-voice broadcast events that anchor into their loaded root's cluster — but only one excluded question requests:

// builds answersByRoot (the anchoring map)
isAgentVoiceBroadcast(m) && m.sessionEventType !== 'question_requested' && rootIds.has(m.threadRootEventId)

// removes the message from visibleMessages
isAgentVoiceBroadcast(m) && rootIds.has(m.threadRootEventId)   // ← no guard

A broadcast session.question_requested with a loaded root would be dropped from the feed by the second and skipped by the first — rendering nowhere while still counting toward unread. That's precisely the shape of #562: counted, but impossible to see.

It's unreachable today only because the server emits question requests without broadcast: true. Nothing enforced that; it was an accident waiting to happen.

The fix

Both call sites now derive from one anchorsIntoRootCluster predicate, so they cannot disagree: a message leaves the feed only if the cluster actually takes it. A broadcast question request stays a row of its own instead of vanishing. The type guard also lets the non-null assertion go.

Verification

The new test fails without the fix — Unable to find an element with the text: Should this include the migration?, i.e. the event rendering nowhere.

Local: pnpm lint · typecheck (7/7) · web 776 · e2e 119/119.

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