fix(ssh): map mkdir-on-existing-dir to AlreadyExists so dir-merge copy works

[zoza1982]

What

Copying a directory onto a remote SFTP directory that already exists failed with Copy failed: …
(the reporter saw Copy failed: not found). This fixes it so a directory-merge copy proceeds.

Root cause

OpenSSH's sftp-server reports mkdir on an existing path as a generic SSH_FX_FAILURE — the
SFTP protocol has no dedicated "already exists" status — and different server builds phrase it
differently ("Failure" on the OpenSSH box I reproduced against, "not found" on the reporter's
DietPi host). The SSH backend surfaced this as an opaque VfsError::Backend, but the transfer
engine's copy_tree only tolerates VfsError::AlreadyExists:

match dst.create_dir(&t).await {
    Ok(()) => outcome.dirs += 1,
    Err(VfsError::AlreadyExists(_)) => {}   // ← only this is tolerated
    Err(e) => return Err(e.into()),         // ← everything else aborts the whole copy
}

Since the destination folder almost always already exists (you're copying into it), every such
copy aborted.

Fix

SftpVfs::create_dir now disambiguates server-agnostically: on a failed mkdir, it stats the
path — if it's already a directory, it returns AlreadyExists (matching every other backend, so the
merge proceeds); otherwise it surfaces the original error unchanged. The extra stat happens only on
the failure path, not the happy path. A collision with an existing file still surfaces the real
error (not masked as AlreadyExists).

Reproduction & testing

• Integration test (tests/copy_repro.rs, CAIRN_IT_SFTP=1): drives the real OpenSSH
  sftp-server as the transfer destination and runs the actual cairn_transfer::run_transfer
  local→SFTP. Before the fix: Err(Vfs(Backend { msg: "Failure: Failure" })). After:
  Ok(TransferOutcome { files: 2, dirs: 1, bytes: 10 }).
• Hermetic unit test: MockSftp made faithful (rejects mkdir on an existing path like the real
  server); create_dir on an existing dir → AlreadyExists, on an existing file → the real
  Backend error.
• Full crate suite green; fmt + clippy --all-targets --all-features -D warnings clean.

Adds dev-deps cairn-transfer + cairn-backend-local to the SSH crate so the engine and backend can
be exercised together against a real server (test-only; no impact on the shipped/lean build).

Follow-up (noted, not in this PR)

map_err in real.rs classifies SFTP errors by matching on the stringified message. Matching
russh-sftp's typed StatusCode instead would be more robust; tracked separately.

Review gates

bug-bot and code-review run on the diff.

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