chore(deps): bump russh 0.45 -> 0.60.1, drop GHSA-f5v4-2wr6-hqmg allowlist (#1851)

[oferchen]

Summary

• Bumps the embedded-ssh transport's russh dependency from 0.45 to 0.60.1, retiring the GHSA-f5v4-2wr6-hqmg / RUSTSEC-2023-0071 Marvin Attack timing side-channel by transitively pulling rsa 0.10 (the constant-time rewrite). Removes the corresponding entry from deny.toml.
• Folds russh-keys (now re-exported under russh::keys), drops the ssh-key and async-trait workspace deps. Fewer crates, smaller dep graph.
• Migrates all consumers in crates/rsync_io/src/ssh/embedded/ to the new API surface (native async fn in traits, PrivateKeyWithHashAlg, AuthResult::success(), Bytes-based Handle::data, etc).

Why

#1851 (P0, beta-blocking) tracks the Marvin Attack allowlist for embedded-ssh. Default builds were unaffected, but cargo deny check advisories had to ignore RUSTSEC-2023-0071 to avoid CI failures. Upstream russh has since cut a 0.60 series that depends on the rewritten rsa 0.10, eliminating the timing side-channel at the source. This change retires the allowlist instead of carrying it indefinitely.

API surface migration (notable breaks)

Area	Before (0.45)	After (0.60)
Async traits	#[async_trait::async_trait]	Native async fn in traits
Key types	russh_keys::key::PublicKey, ssh-key	russh::keys::{PublicKey, PrivateKey}
Pubkey auth	authenticate_publickey(user, Arc<KeyPair>)	authenticate_publickey(user, PrivateKeyWithHashAlg::new(Arc::new(key), None))
Auth result	bool	AuthResult with .success()
Channel write	Handle::data(id, CryptoVec::from_slice(&data))	Handle::data(id, data) (Vec into Bytes)
Server start	server.run_on_socket(cfg, &listener).await?	server.run_on_socket(cfg, &listener).await (no Result)
Fingerprint	key.fingerprint()	key.fingerprint(HashAlg::Sha256)
Algorithm name	key.name()	key.algorithm()
Mock server reject	Auth::Reject { ... }	Auth::reject() constructor

Files touched

• Cargo.toml - workspace pin russh = "0.60.1"; remove async-trait, russh-keys, ssh-key.
• Cargo.lock - regenerated via cargo update -p russh. Major movement: rsa 0.9.10 -> 0.10.0-rc.16, adds internal-russh-forked-ssh-key, removes standalone russh-keys / ssh-key.
• deny.toml - drop RUSTSEC-2023-0071 from advisories.ignore.
• crates/rsync_io/Cargo.toml - drop async-trait/russh-keys/ssh-key from the embedded-ssh feature; add rand = "0.10" dev-dep for test keypair generation (matches russh 0.60's rand version).
• crates/rsync_io/src/ssh/embedded/handler.rs - migrate SshClientHandler to native async fn check_server_key; use russh::keys::{HashAlg, PublicKey, known_hosts}.
• crates/rsync_io/src/ssh/embedded/auth.rs - rewrite agent / pubkey / password auth paths against the new API; rewrite the mock SSH test server (no #[async_trait], Auth::reject(), PrivateKey::random(&mut rand::rng(), Algorithm::Ed25519)).
• crates/rsync_io/src/ssh/embedded/connect.rs - drop CryptoVec wrapping; pass Vec<u8> directly to Handle::data.
• docs/audits/ssh-transport-timeout-coverage.md - update russh_keys::load_secret_key reference to russh::keys::load_secret_key.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --no-deps -- -D warnings
• cargo nextest run --workspace --all-features (full CI matrix)
• cargo deny check advisories - must pass without the RUSTSEC-2023-0071 allowlist.
• Required CI checks: fmt+clippy, nextest (stable), Windows (stable), macOS (stable), Linux musl (stable).
• Embedded-SSH integration tests (authenticate_pubkey_succeeds, authenticate_password_succeeds, host-key verification suite) exercise the migrated API end-to-end.

Closes #1851