* feat(ecstore): add on-demand migration bucket config model
Introduce OnDemandMigrationConfig (deny_unknown_fields, version 1) with typed validation, credential redaction, a secret-free Debug impl, and the OnceLock publish hook the runtime registers into. Exported through the api facade.
* feat(ecstore): persist on-demand migration config in bucket metadata
Store the config as a RustFS extension entry (on-demand-migration.json) with its update time in .metadata.bin, add the typed BucketMetadataSys accessor, and publish the config through the hook on every cache-install path alongside the durability sync.
* test(e2e): rename stall timing variable flagged by typos
* test(storage): heap-pin the RestoreObject usecase future in the generation guard test
