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.

1 files changed, 1 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 162596d3..674fa358 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,6 +20,7 @@ async-trait = "0.1.89"
 base64 = "0.22.1"
 chrono = { version = "0.4.44", features = ["serde"] }
 fastrand = "2.4.1"
+futures = "0.3.32"
 indexmap = { version = "2.14.0", features = ["serde"] }
 jsonschema = { version = "0.46.6", default-features = false }
 md5 = "0.7.0"