put: degrade to the anonymous FTP queue when the SSH transport fails

pkh put only spoke SFTP to the PPA queue, so a failure of the SSH
transport itself (TCP, banner exchange) failed the upload even though
dput happily pushes the same files: its plain ppa: profile goes over
the anonymous FTP queue of ppa.launchpad.net, the same destination
over another port.

Classify the SSH connection failures: Transport (the connection never
came up: resolution, TCP, banner or key exchange) degrades to that FTP
queue — the upload order (payload first, .changes last), the
reverse-order DELE cleanup of a failed upload and the per-chunk
progress reporting all mirror the SFTP path, sharing cleanup_list.
Refused failures (host key not accepted, no matching authentication)
stay errors: silently switching transport would bypass the refusal.

The FTP client is suppaftp's blocking stream, with the time bounds it
does not carry by itself: the control channel's reads and writes, the
data channel's writes and connect (through a custom passive stream
builder), and the NAT workaround for PASV replies announcing an
unroutable address. The queue endpoints (host, port) join
data/launchpad.yml next to the SFTP ones, and the FTP transport is
covered by unit tests against an in-process fake queue plus a live
control-channel handshake with the real server (ignored, network).

src/launchpad.rs

@@ -41,6 +41,11 @@ struct LaunchpadData {
     ssh_host: String,
     /// Port of the PPA SFTP upload server
     ssh_port: u16,
+    /// Host of the PPA upload queue over anonymous FTP (the transport
+    /// `pkh put` degrades to when the SSH connection never comes up)
+    ftp_host: String,
+    /// Port of the anonymous FTP upload queue
+    ftp_port: u16,
     /// Upload queue incoming directory template (`{owner}`/`{ppa}`)
     incoming_template: String,
     /// PPA package-content (apt repository) URL template
@@ -53,6 +58,13 @@ embed_data! {
     static ref LAUNCHPAD_DATA: LaunchpadData = "../data/launchpad.yml"
 }
 
+/// The PPA upload queue over anonymous FTP (host, port): the transport
+/// dput-ng's plain `ppa:` profile pushes over, and the one `pkh put`
+/// degrades to when the SSH connection itself never comes up.
+pub(crate) fn ppa_ftp_queue() -> (String, u16) {
+    (LAUNCHPAD_DATA.ftp_host.clone(), LAUNCHPAD_DATA.ftp_port)
+}
+
 /// Base URL of the Launchpad REST API
 fn api_base() -> &'static str {
     &LAUNCHPAD_DATA.api_base
@@ -422,6 +434,13 @@ mod tests {
         assert_eq!(target.login, None);
     }
 
+    /// The anonymous FTP fallback queue resolves from the same data the
+    /// dput-ng `ppa:` profile uses.
+    #[test]
+    fn ppa_ftp_queue_resolves() {
+        assert_eq!(ppa_ftp_queue(), ("ppa.launchpad.net".to_string(), 21));
+    }
+
     #[test]
     fn ppa_target_rejects_missing_separator() {
         assert!(ppa_target("just-a-name").is_err());