Last gap in the filer-backed ListFunc. A directory at .uploads/<id>
carrying ExtMultipartObjectKey is the MPU init record; emit one
bootstrap.Entry with IsMPUInit=true and DestKey set to the user's
intended path. The walker's MatchPath uses DestKey for prefix
matching; the WalkerDispatcher uses it for the LifecycleDelete RPC's
ObjectPath. .uploads/<id> directories without the extended key are
mid-write before metadata landed and stay skipped.
isMPUInitDir is upgraded from the path-shape-only stub to the full
shape + extended-attr check that mirrors router.mpuInitInfo and
scheduler/bootstrap.go's same-named helper.
Tests pin: valid init record emits with the right DestKey, missing
ExtMultipartObjectKey skips the directory.
Chris Lu
e3c5f624b9