new: add pkh put, a native dput replacement for PPA uploads

Upload built source packages over SFTP with host-key verification
(Launchpad fingerprints pinned in host_keys.yml, ask-to-accept
otherwise), Launchpad account discovery (git config lp.user), and
pre-flight checks the upload queue itself never does: changes file
discovery/validation, PPA existence via the Launchpad API, target
series validity, and debian/control Section validity (sections
bundled in distro_info.yml). Upload log prevents duplicate uploads
unless --force.

src/put/target.rs

@@ -0,0 +1,26 @@
+//! Generic upload target vocabulary for `pkh put`. Concrete targets are
+//! built by their service modules (Launchpad — [`crate::launchpad`] —
+//! today, archive targets later); this module only defines what any upload
+//! target looks like and how the SSH username resolves when the service
+//! does not pin one.
+
+/// A concrete SFTP upload destination.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct UploadTarget {
+    /// Hostname to connect to.
+    pub fqdn: String,
+    /// TCP port (always 22 for now; SFTP only).
+    pub port: u16,
+    /// Default SSH username, when the service has one; `None` means the
+    /// username resolves per machine (service-specific discovery — git
+    /// configuration for Launchpad — then SSH configuration `User`, then
+    /// the local user name).
+    pub login: Option<String>,
+    /// Incoming directory on the server, relative to the SFTP root. For
+    /// Launchpad targets this is a literal `~user/...` path: the upload
+    /// queue is write-only, so the path names the destination rather than a
+    /// browsable directory.
+    pub incoming: String,
+    /// Human-readable target name for logs and the upload record.
+    pub label: String,
+}