Adds dailyrun.WalkBuckets — the composable driver the handler-side
WalkerFunc will call. Iterates a bucket list, wraps the supplied
bootstrap.ListFunc with a per-shard filter (Path for non-MPU, DestKey
for MPU init), and runs bootstrap.Walk per bucket using the supplied
Dispatcher. First bucket error wins; remaining buckets log and run to
completion so one filer flake doesn't kill the shard.
Composable rather than monolithic so callers and tests can swap parts:
production uses a filer-backed ListFunc + WalkerDispatcher; tests use
bootstrap.EntryCallback + a stub. The filer-backed ListFunc is the
next commit.
Tests pin: shard filter routes only matching entries, MPU shard uses
DestKey not the .uploads/<id> path, single-bucket error propagates
while other buckets still run, ctx cancellation short-circuits between
buckets, nil guards on view/list/dispatch.