Tail-position rewrite: walk_cgroup + entry_presence (unblocks D2 floor)

[briansrls]

Auto-opened by session-dashboard for session lively-newt-303.
Pushing to session/lively-newt-303 advances this PR.

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[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-09T01:35:42.378337Z	395b226	Draft marked ready

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[gunbai-bot]

Closing: this PR was auto-opened from an unverified draft (not compiled, not emitted, no claim run, per its author's handoff). The same fix is being finished, with receipts, by sharp-heron-165, who will open the PR that unblocks #13217.