Add port forwarding: local, remote and dynamic, per terminal session

A host can now carry port forwards (-L, -R, -D), saved inside its
encrypted payload as RelayTicketRequest.PortForwardId already assumed,
so they sync and merge per forward like tags and pinned paths. The host
schema moves to version 8 only for hosts that carry forwards.

ISshConnection gains StartForwardAsync over SSH.NET's forwarded ports.
Every listener binds loopback and there is no bind-address field: a
forward is something any member of a shared keychain can edit. A remote
forward's target is resolved here, IPv4 first, rather than by SSH.NET's
constructor, which takes the first DNS answer and can send ::1.

Forwards start with every terminal opened to the host and stop with its
connection; a reconnect starts them again. The session sidebar lists
them with state, failure reason and an on/off switch, the status bar
counts the listening ones, and the host editor and detail pane gain a
PORT FORWARDS section. A forward that cannot start fails alone.

The phone does not start forwards yet: it has no surface showing what
is listening, and its More screen now says so. No byte counts, since
SSH.NET keeps none.

tests/DodoSSH.Client.Terminal.Tests/TerminalWorkspaceTests.cs

@@ -599,6 +599,53 @@ public sealed class TerminalWorkspaceTests
         await WaitUntilAsync(() => Volatile.Read(ref reattachedCount) > 0);
     }
 
+    // ---- Port forwards ----
+
+    [Fact]
+    public async Task AForward_RunsOverTheConnectionOfTheSessionItNames()
+    {
+        var connections = new FakeConnectionFactory(blockShellReads: true);
+
+        await using var workspace = CreateWorkspace(connections);
+
+        await workspace.OpenSessionAsync(
+            Request(), TerminalSize.Default, progress: null, TestContext.Current.CancellationToken);
+        var second = await workspace.OpenSessionAsync(
+            Request(), TerminalSize.Default, progress: null, TestContext.Current.CancellationToken);
+
+        var spec = new SshPortForwardSpec(SshPortForwardKind.Local, 5432, "db.internal", 5432);
+
+        await using var forward = await workspace.StartForwardAsync(
+            second, spec, TestContext.Current.CancellationToken);
+
+        forward.Spec.ShouldBe(spec);
+        connections.Connections[0].Forwards.ShouldBeEmpty();
+        connections.Connections[1].Forwards.ShouldBe([spec]);
+    }
+
+    [Fact]
+    public async Task AForwardOnASessionThatIsNotRunning_IsRefused()
+    {
+        var connections = new FakeConnectionFactory(bytesPerShell: 0);
+
+        await using var workspace = CreateWorkspace(connections);
+
+        var ended = await workspace.OpenSessionAsync(
+            Request(), TerminalSize.Default, progress: null, TestContext.Current.CancellationToken);
+
+        await WaitUntilAsync(() => !workspace.IsSessionLive(ended));
+
+        var spec = new SshPortForwardSpec(SshPortForwardKind.Dynamic, 1080);
+
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await workspace.StartForwardAsync(ended, spec, TestContext.Current.CancellationToken));
+
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await workspace.StartForwardAsync(999, spec, TestContext.Current.CancellationToken));
+
+        connections.Connections[0].Forwards.ShouldBeEmpty();
+    }
+
     // ---- Helpers ----
 
     /// <remarks>