srv3 error-type cascade: establish or refute a FOURTH binding channel (variant resolution)

[briansrls]

Auto-opened by session-dashboard for session bold-stag-694.
Pushing to session/bold-stag-694-repair2 advances this PR.

Worker attestation

Before flipping this PR to ready for review, confirm each item:

• Title describes the change (not the session id or branch).
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• Tests run: name the command (e.g. npm test, cargo test) and the result.
• If this closes a work item, the body contains a Closes #N directive.
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Summary

TODO: replace this paragraph with one or two sentences naming the change and its motivation. Reviewers read this first.

Test plan

• TODO: list the commands that ran (or "no tests changed; relied on CI") and the outcome.