fallback-arm census closeout — lens+register enrolled; Class F child Accept or owner confirmed

[briansrls]

Auto-opened by session-dashboard for session silent-carp-216.
Pushing to session/silent-carp-216 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).
• PR body summarises what and why (replace the TODO below).
• 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.
• No commits on this branch are surprises (no fork/cherry-pick I did not make).
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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.

[gunbai-bot]

Superseded by #7260 (session/bright-newt-31).

This child session forked a parallel lens name (fallback_arm_classify / FallbackArmClassify) against the authority landing on #7260 (v2.lens.fallback_arm_census, FallbackArmCensus). Per parent closeout directive: archive this PR; census closeout continues on #7260.

Class F half of the closeout title is obsolete here (perturb-attribution → #7216; interpreter-defect half unowned per plan §6).