The docker-options DOCKER_OPTIONS_<PHASE>.migrated sentinels and the storage data/storage-registry/migrations/<app> flag files are now stored as per-app properties (`migrated-<phase>` and `legacy-mounts-migrated` respectively), so they survive a property-store backup/restore and align with the existing `migrated-from-files`, `env-migrated`, and `nginx-conf-sigil-migrated` patterns. The new markers are written only when a non-empty legacy source was actually migrated, distinguishing apps that never had legacy state from apps that did and were drained. A one-cycle upgrade-cycle conversion drains any leftover filesystem sentinels into the new properties and removes the files; for docker-options the conversion deliberately runs before the global `migrated-from-files` short-circuit so installs upgrading from the previous release still get their `.migrated` files converted.
The bats test for migration idempotency invoked the install binary directly via sudo, which kept hitting fresh missing env vars on each iteration (DOKKU_LIB_ROOT, then PLUGIN_PATH, etc). The dokku launcher script exports a chain of vars that the install path reads via common.MustGetEnv, and mirroring that chain in a bats test is fragile. Migration is pure file IO plus a property marker - perfect for a Go unit test that isolates itself with t.TempDir and t.Setenv. The new test covers parsing comments and blank lines, the marker-based no-op on re-run, and the rule that a manually re-created legacy file is left untouched after the marker is set.
