refactor(loop): drive wait() promises concurrently via FuturesUnordered

Loop::wait already had the target signature and a TODO(phase-c-promise)
marker noting it drove promises serially; swap the for-loop for
FuturesUnordered so all promises are polled together instead of one at
a time, keeping the "remember only the first error" semantics.

This adds the first real use of the futures dependency (already present
in Cargo.toml/Cargo.lock from earlier prep work, now finally consumed),
so those lockfile/manifest changes land in this commit.

Real overlap still doesn't happen yet: each promise (HttpDownloader::add/
add_copy etc.) resolves through a blocking bridge (curl_runtime()/
sync_executor::block_on) that fully occupies the thread until it
settles, so this is groundwork for once a single top-level Runtime
replaces those bridges. Updated the TODO(phase-c-promise) comment to
reflect that.

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