8967ba7cb4
exportAdapter.GetStackHDDMounts now returns stacks.ExportDataMounts — the
${HDD_PATH} binds UNIONed with the ${USERDATA_PATH} ROOT (captured at the
root, not per-bind, so the manifest's basename keying round-trips through the
existing import mapping without touching restore). Containment-aware dedupe
both directions. The backup-side stackAdapter is intentionally unchanged.
Red-proof: pre-fix behavior fails TestExportDataMounts_UserdataConvention/
MixedBindsUnion/LiteralUserdataBindDeduped (run->fail->revert recorded).