docs: load extensionless mirror receiver probe

[xxvcc]

Summary

• Load the root-owned extensionless receiver candidate with an explicit SourceFileLoader in the production filesystem probe.
• Keep the probe bound to the exact candidate bytes that will be atomically installed.

Testing

• The complete documented Bash block passes bash -n.
• The exact embedded Python probe was executed as ltamirror against /www/wwwroot/dl.ll.cd/linux-temp-admin using an extensionless root-owned 0755 candidate.
• The probe proved successful no-replace rename, EEXIST refusal without byte changes, and left no candidate or document-root probe residue.
• Go, shell, workflow, account, sudoers, and systemd behavior is unaffected.

Safety Notes

• No credentials, private keys, hostnames, IP addresses, account data, or authorization material were added.
• This changes release-operator documentation only; the receiver code merged in fix: make mirror publication crash-recoverable #31 is unchanged.