Move files to and from a host over SFTP

M2's file transfer, built bottom-up: an SFTP session on the SSH layer, a
transfer queue in a project of its own, and the two-pane browser the design
asked for replacing the screen that said it did not exist. Remote listings
carry names, sizes, modification times and a real drwxr-xr-x — nothing in this
repository could render a POSIX mode before — and the queue moves one file at a
time with progress, throughput and resume.

The design import assumed this would be an SFTP subsystem channel on
ISshConnection, beside the shell on a transport that is already up. SSH.NET
does not offer that: SftpClient derives from BaseClient and owns its own
transport, and there is no supported way to hand it an SshClient's session. So
file transfer opens a second authenticated connection, and it is named for
that rather than dressed up as a channel — OpenSftpAsync is on
ISftpSessionFactory, not on a connection. The difference is visible to a user:
the host records a second login, and a host whose password is typed each time
asks for it again on this screen. It goes through the same host key gate, the
same pin and the same two refusals a shell does, so a fingerprint approved for
a terminal is approved here and one approved here reaches the other machines
with the next sync. docs/design-import-gaps.md is corrected, and marked as the
one row where what shipped differs from what it predicted.

Nothing is written at its final name until it is complete. Every transfer goes
to a .dodossh-part file beside its destination and is renamed into place at the
end, so an interrupted transfer can never be mistaken for a finished one —
which matters most for what this screen is actually for, which is copying a
build artefact onto a server and then running it. A destination that already
exists is refused outright rather than overwritten: the queue has no way to
ask, and silently replacing a file somebody's process is serving is the worse
of the two failures. The remote pane has DELETE and MKDIR so that refusal is
not a dead end. A test against the container pins the assumption underneath all
of this — that SFTP's rename does not clobber.

Resume works within a run of the application and not across a restart, and the
limit is deliberate rather than unfinished. Nothing records which source wrote
a part file, and resuming one on the strength of its name matching is how a
corrupt artefact gets delivered with nothing reporting a failure; a part file
found at startup is started over. Making it survive a restart needs the
preferences store this client still has not got. The offset a resume starts at
is the part file's own length rather than the transfer's recorded progress: a
cancellation can land between a write completing and the counter moving, and
only one of those two is a fact about the bytes that are there.

The queue and its connection outlive a lock, as shells do. LockAsync already
argues that locking must not destroy work in flight — it is what somebody does
when they walk away from the machine, which is exactly when a long transfer is
most likely to be running — so TransfersViewModel is created once and the vault
is attached on unlock and detached on lock. What locking takes is the host
list, and it has to: those rows carry decrypted secrets.

DodoSSH.Client.Transfer is a new project rather than more of Client.Ssh. The
two answer different questions — one is about reaching a host, the other about
moving bytes and what to do when moving them stops halfway — and this is the
only client project that deliberately touches the local filesystem.

Three defects the tests found, none of which review would have. SftpPath.Name
answered an empty string for the root. NavigateRemoteAsync wrapped itself in
the busy guard, so navigating from inside another command did nothing at all
and the remote pane simply stayed empty after connecting, with no failure
anywhere to explain it. And opening an SFTP session per test made two
handshakes per test — this client learns a host key by being refused — which
pushed the SSH assembly past sshd's MaxStartups and failed a different few
unrelated tests each run; the session is shared through the fixture now, with
the reason written where the next person will hit it.

1004 tests green across 18 projects, 24 of them new: the SFTP subsystem against
the OpenSSH container, the queue against a real temporary directory and a fake
host, and three more layout measurements because a screen this window has never
laid out is a screen never checked.

Not verified: the screen has not been looked at running. The layout harness
measures it at the window's minimum in three shapes, which is the class of
defect that has shipped here before, but reaching it in the application needs
the compose stack, the migrations, the API and a browser sign-in. What is still
absent — the status bar's transfer count, dragging between the panes,
transferring a directory, and sftp over a bastion — is in
docs/design-import-gaps.md.

docs/design-import-gaps.md

@@ -18,9 +18,13 @@ M2 and M3 arriving in a design before it arrives in the code.
 Three facts explain nearly every row below.
 
 **An SSH connection here opens exactly one channel.** `ISshConnection` offers `OpenShellAsync` and nothing
-else (`src/DodoSSH.Client.Ssh/SshConnection.cs`). No SFTP subsystem, no port forwarding, no ProxyJump. That
-one fact removes the whole file-transfer screen, the `FORWARDS` chip, the status bar's port list, and every
-`via bastion-eu` in the design.
+else (`src/DodoSSH.Client.Ssh/SshConnection.cs`). No port forwarding, no ProxyJump. That one fact removes
+the `FORWARDS` chip, the status bar's port list, and every `via bastion-eu` in the design.
+
+It used to remove the file-transfer screen too. M2 did not lift the restriction — it worked around it:
+file transfer is a *separate connection* rather than a second channel, because SSH.NET's `SftpClient` owns
+its own transport. See [File transfer](#file-transfer-the-designs-sftp-screen), which is the one place in
+this document where what shipped differs from what the row predicted.
 
 **Teams are schema and nothing else.** The `team` and `team_membership` tables exist from the first
 migration, with entities in `DodoSSH.Domain/Teams.cs` and a `TeamRole` enum — and no endpoint reads or
@@ -50,7 +54,7 @@ protocol rather than a protocol change.
 | `⌘K` command palette running commands | client-domain | A snippet or saved-command item type (`SyncEntityType.Snippet = 8` is reserved). | Ctrl+K opens a real host search that connects on Enter. The box says "search hosts", not "search hosts · run command". |
 | Status bar `· via bastion-eu` | client-ssh | Jump-host execution. See below. | Omitted. |
 | Status bar port forwards | client-ssh | Port forwarding. See below. | Omitted. |
-| Status bar `sftp · 2 transfers` | client-ssh | File transfer. See below. | Omitted. |
+| Status bar `sftp · 2 transfers` | client-app | Nothing now — file transfer is built. What is missing is the count reaching the status bar, which is a screen away from where the queue lives. | Omitted from the status bar. The queue itself is on the FILES screen, with a row per transfer. |
 | Status bar `locks in 09:41` | client-app | An idle auto-lock. See preferences below. | Omitted. |
 | IBM Plex Mono / IBM Plex Sans | ui | Shipping the font files as `AvaloniaResource` and registering them. The design loads them from Google Fonts, which a desktop app cannot. | Inter (already embedded) for prose, and the system monospace stack the terminal already names. Named once in `App.axaml` as `MonoFont`, so the substitution is reversible in one place. |
 | `⌘K`, `⌥↵` | ui | Nothing; the design is Mac-flavoured. | `CTRL K`. Development is Windows-only today (`docs/platform-flags.md`). |
@@ -86,18 +90,36 @@ caption buttons and window title drawn on top of the application's own — two s
 
 ## File transfer (the design's SFTP screen)
 
-Nothing on this screen exists. It is listed in the nav rail and reaches a screen that says so, naming the
-milestone and what is missing — see `ShellScreen` for why it is not simply dropped from the rail.
+**Built in M2.** The screen ships: two directory panes, a breadcrumb trail on each, and a queue that moves
+one file at a time with progress, throughput and resume. What follows is what it does *not* do, and one
+thing this document got wrong before it was built.
 
-| Design element | Layer | What it would take |
+**The correction.** The row below used to say an SFTP subsystem channel on `ISshConnection` was what it
+would take. SSH.NET does not offer that: `SftpClient` derives from `BaseClient` and owns its own transport,
+and there is no supported way to hand it a session an `SshClient` already has. So the transfers screen
+**opens a second authenticated connection** to the host rather than a second channel on the terminal's. That
+is visible to a user — the host records a second login, and a host whose password is typed each time asks
+for it again on this screen — so it is named for what it is: `ISftpSessionFactory.OpenSftpAsync` is a
+connect, and it goes through the same host key gate, the same pin and the same two refusals a shell does.
+
+| Design element | Layer | What ships |
 | --- | --- | --- |
-| SFTP itself | client-ssh | An SFTP subsystem channel on `ISshConnection`. There is no `SftpClient`, `ScpClient` or transfer type anywhere in `src/`. |
-| Remote listing with `NAME/SIZE/MODIFIED/PERMS` | client-ssh | The channel, plus a listing record and a POSIX mode formatter — nothing in the repo formats a `drwxr-xr-x`. |
-| Local listing | client-app | The App project contains no `System.IO` usage at all. The only paths this client knows are its own two files. |
-| Path breadcrumbs, per-host last directory | client-storage | Navigation state for two panes, and somewhere to persist it. There is no settings table. |
-| Transfer queue, progress, throughput | client-ssh | A transfer engine. **Do not reach for `CreditWindow`** — that is a 256 KiB flow-control window for terminal output, not a transfer primitive. |
-| `resume supported` | client-ssh | Offset-based reads and writes, plus partial-transfer bookkeeping that survives a restart. |
-| `sftp over bastion-eu` | client-ssh | Jump hosts, as above. |
+| SFTP itself | client-ssh | `ISftpSession` over SSH.NET's `SftpClient`: listing, stat, offset-based read and write, mkdir, delete and rename. Delete is deliberately **not** recursive. |
+| Remote listing with `NAME/SIZE/MODIFIED/PERMS` | client-ssh | All four. `PosixMode` renders `drwxr-xr-x` from the bits SFTP hands over; setuid, setgid and sticky are not shown, because SSH.NET does not surface them and `rwx` where `rws` is true would be worse than nothing. |
+| Local listing | client-transfer | `LocalDirectory`, which is where this client's `System.IO` now lives. `PERMS` is blank on the local side rather than filled with a plausible-looking POSIX mode that is not a fact about a file on Windows. |
+| Transfer queue, progress, throughput | client-transfer | `FileTransferQueue`. One transfer at a time, so the rate on a row is the rate of the link rather than a share of it. Throughput is measured over a half-second window, not averaged since the start. |
+| `resume supported` | client-transfer | **Within a run of the application.** Every transfer writes to a `.dodossh-part` file beside its destination and is renamed into place at the end, so an interrupted one can never be mistaken for a finished one, and `RESUME` carries on from the part file's own length. A part file found at startup is *not* resumed: nothing records what wrote it, and resuming on the strength of a name matching is how a corrupt artefact gets delivered with nothing reporting a failure. Making it survive a restart needs the preferences store this client has not got — see below. |
+
+| Design element | Layer | What it would take | What ships instead |
+| --- | --- | --- | --- |
+| Per-host last directory | client-storage | Somewhere to persist two panes' navigation state. There is still no settings table. | The remote pane opens on the account's home directory, which the server canonicalises during the handshake; the local pane opens on the user profile. |
+| `sftp over bastion-eu` | client-ssh | Jump hosts, as above. `HostSecret.JumpHostIds` is still stored, synced, merged and read by nothing. | Omitted. |
+| Overwriting a file that is already there | ui | A prompt, which means a modal this window has no idiom for. | Refused, with the name that is in the way. The remote pane has DELETE and MKDIR so the refusal is not a dead end. |
+| Dragging between the panes | ui | Drag-and-drop between two `ListBox`es, plus a drop target that is a directory rather than a row. | Two arrow buttons between the panes, pointing at the pane the file is going to. |
+| Transferring a directory | client-transfer | Recursive enumeration on both sides, and a policy for what a partial directory means. | One file at a time. A directory cannot be selected as a transfer source. |
+
+**Not to be reached for:** `CreditWindow` is a 256 KiB flow-control window for terminal output, not a
+transfer primitive. The queue does its own 64 KiB copy loop and shares nothing with the terminal data plane.
 
 ---