refactor: redundancy getter exhaust

[nugaon]

Small cleanup of pkg/replicas/getter.go prompted by a beekeeper CI failure where the return nil, nil at the bottom of Get was somehow reached, causing a nil-panic downstream in joiner.New.

Changes:

• NONE short-circuit: NewGetter with redundancy.NONE now returns g.Getter.Get directly. The replica machinery (goroutines, ticker, replicator) is pointless for level 0, and emitting ErrSwarmageddon when the chunk is simply not found at level 0 was semantically wrong.
• return nil, nil → errGetterExhausted: the trailing return is structurally unreachable (the replicator exhausts exactly at the last loop iteration, turning it into an indefinite wait on resultC/errc; the loop always exits via success or Swarmageddon). Renamed to a named sentinel with a matching comment so the intent is clear if it ever surfaces again.
• Comment above loop: explains the batch-doubling design (2, 2, 4, 8, 16 for PARANOID) and why the loop cannot fall through to the bottom.
• Test: level=0 all-fail case no longer asserts ErrSwarmageddon; the raw error is returned via the short-circuit.