move stored_secret from swarm-queue-client to swarm-secret-client

The module is a rename (97% similar); everything else in the diff follows
from the move:

- stored_secret.rs: private `chain()` helper, the 8-line copy of the queue
  crate's `chain` the module called as `crate::chain`; the queue crate's
  public `chain` stays.
- stored_secret.rs: `Duration::from_mins(5)` in `MAX_REREAD_DELAY` is
  unchanged moved code, not a new use.
- swarm-secret-client Cargo.toml: gains futures-util, tokio and tracing as
  dependencies and tokio `test-util` plus tracing-subscriber as dev
  dependencies; the module and its tests use them.
- swarm-queue-client Cargo.toml: drops futures-util and the tracing-subscriber
  dev dependency, which only the moved module used; its tokio dev comment is
  reworded to the tests that remain.
- Cargo.lock: the same dependency moves, nothing else.
- swarm-queue-client lib.rs: imports `swarm_secret_client::stored_secret`,
  and its tests use a small local `Store` in place of the moved module's
  test-only `Fake`, which is no longer reachable.
- callers (hive-c0re, swarm-builder, swarm-ci-metrics, swarm-claude-login,
  swarm-controller, swarm-nats-auth, swarm-review-classifier): import path
  only; no re-export is left behind.

swarm-secret-client/Cargo.toml

@@ -4,6 +4,9 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
+# `Shared`, so every caller refused while a secret re-read runs waits on that
+# one read (`stored_secret`).
+futures-util.workspace = true
 # `Identity` is re-exported by vaultrs, but the identity is built here (from
 # the `BAO_*` files) rather than by its env defaults, so the dependency is
 # direct rather than incidental.
@@ -21,6 +24,9 @@ serde.workspace = true
 serde_json.workspace = true
 strum.workspace = true
 thiserror.workspace = true
+# `stored_secret` spawns its re-reads and times them out.
+tokio.workspace = true
+tracing.workspace = true
 vaultrs.workspace = true
 
 [lints]
@@ -31,3 +37,9 @@ workspace = true
 # test would pass with `install_crypto_provider` doing nothing.
 [dev-dependencies]
 rustls = { workspace = true, features = ["ring"] }
+# `test-util` for `#[tokio::test(start_paused = true)]`: the re-read backoff
+# tests run on paused time.
+tokio = { workspace = true, features = ["test-util"] }
+# Capturing what a re-read logs, to check it names the client and never the
+# secret.
+tracing-subscriber.workspace = true