`rad lfs fetch` still exists for single-object/manual use, but
`radicle-lfs-transfer` used to spawn a fresh one per object during
`git lfs pull`/checkout -- for a private repo, each one independently
loaded the signer, so N files meant N passphrase prompts. Same bug
class `rad lfs precommit` already fixed for commits, unfixed here
until now.
Unlike precommit (which knows every staged file upfront and can read
stdin until EOF), fetch can't collect a batch before starting:
git-lfs's custom-transfer protocol requests objects one at a time.
`rad lfs fetch-batch` instead stays alive for as long as its caller
keeps its stdin open, answering one "<oid> <size> <out-path>" request
at a time with a JSON response line, reusing the same unwrapped
signer across all of them -- paired with radicle-lfs-transfer's new
FetchWorker, which spawns this once per transfer session instead of
once per object.
Also factors `fetch::run`'s core logic out into `fetch_object`,
taking a `&mut Option<MemorySigner>` the same way `store::store_object`
already does, so both the single-shot `rad lfs fetch` and the new
batch worker share one implementation.
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