Relax ancestor ACL requirement for runtime dir (fixes #1621)

[Carnival-z]

Summary: Allow ancestors (like /private/tmp on macOS) that carry allow-type ACLs so the daemon can create per-user runtime directories (/private/tmp/cbm-daemon-). The final runtime directory is still hardened: owner == current user, chmod 0700, and extended ACLs cleared.

Security: Only ancestor observation rules were relaxed for compatibility. Final runtime dir hardening (0700 + ACL clear) remains unchanged.

Testing suggestions:

macOS: verify daemon can create /private/tmp/cbm-daemon- when /private/tmp carries allow-ACLs; verify runtime dir is 0700 and ACLs cleared.
Ensure socket/lock/log files remain 0600 and ACLs cleared.
Fixes: #1621

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

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Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

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