Keep safe-parent enforcement and private-file IO unchanged on Windows. Restore the original POSIX mkdir/chmod calls, supplied-parent hardening, Pi stash directory refusal and enrollment-only ancestor traversal (including private groups and sticky directories).

Restore directory/ancestry tests verbatim rather than preserving the 0.15.0 reversed expectations; retain the separate cooperative lease-fence bug fix and its observer tests by explicit reviewer decision. Packaging tests retain the lazy Windows adapter allowance. Retarget enrollment mutations and add a foreign-owned group-writable ancestor store regression with a refusing positive control.