Take a returning renderer's socket over instead of refusing it

One attach per process was WebView2's truth, not Android's: the phone
kills the WebView's renderer independently of the app process, the page
reloads, and its fresh socket was answered 409 by a guard that never
reset — with no way back short of restarting the app. Only our own page
knows the token, so a second valid upgrade is that page returning; it
now displaces the old socket, which may never notice it is dead on its
own, since a killed renderer sends no FIN.

A send into the dead socket also no longer escapes as a fault. It used
to unwind the pump's flush loop, after which nothing drained the credit
window and the still-live shell froze behind it for good — including
the BCL quirk where such a send surfaces as an OperationCanceledException
nobody's token asked for.

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

@@ -142,15 +142,96 @@ public sealed class TerminalDataPlaneTests : IAsyncDisposable
             await ConnectAsync(origin: "https://evil.example"));
     }
 
+    /// <remarks>
+    /// The truth this replaced: a second valid attach used to be a 409, on the theory that one renderer
+    /// lives for the whole process. Android's WebView does not honour that theory — its renderer process is
+    /// routinely killed and the page reloads with a fresh socket — so a second valid attach is now a
+    /// takeover. This asserts both halves: the newcomer gets the connection, and the displaced socket
+    /// actually goes rather than lingering as a phantom nothing is reading from.
+    /// </remarks>
     [Fact]
-    public async Task ASecondRenderer_IsRejected()
+    public async Task ASecondRenderer_TakesOver_AndTheFirstSocketIsDropped()
     {
         Start();
 
         using var first = await ConnectAsync();
         first.State.ShouldBe(WebSocketState.Open);
 
-        await Should.ThrowAsync<WebSocketException>(async () => await ConnectAsync());
+        using var second = await ConnectAsync();
+        second.State.ShouldBe(WebSocketState.Open);
+
+        // The first socket was aborted rather than closed gracefully — Abort skips the close handshake
+        // entirely, so there is no Close frame for this side to see coming. What a receive on it sees
+        // instead is the connection simply gone, which the client surfaces as an exception rather than as
+        // a state that quietly flips on its own; nothing here reads from the socket otherwise, so the
+        // state alone would not move.
+        var firstBuffer = new byte[16];
+        await Should.ThrowAsync<Exception>(async () =>
+            await first.ReceiveAsync(firstBuffer.AsMemory(), TestContext.Current.CancellationToken));
+
+        await using var session = new FakeShellSession(bytesToProduce: 64);
+        await using var pump = CreatePump(session);
+        plane.Register(SessionId, pump);
+
+        var run = pump.RunAsync(TestContext.Current.CancellationToken);
+
+        var opened = await ReceiveAsync(second);
+        opened.Opcode.ShouldBe((byte)TerminalServerOpcode.SessionOpened);
+
+        var output = await ReceiveAsync(second);
+        output.Opcode.ShouldBe((byte)TerminalServerOpcode.Output);
+        output.Payload.Length.ShouldBe(64);
+
+        await SendAsync(
+            second,
+            (byte)TerminalClientOpcode.Acknowledge,
+            TerminalFrame.CreateAcknowledgementPayload((uint)output.Payload.Length));
+
+        await run;
+    }
+
+    /// <remarks>
+    /// <para>
+    /// The other half of the takeover: a renderer process that dies without a close handshake — which is
+    /// what a killed Android WebView actually does, no FIN, nothing — must not fault the send path. A
+    /// faulted send would propagate into <see cref="TerminalSessionPump"/>'s flush loop and freeze a live
+    /// session; see <see cref="TerminalDataPlane.SendAsync"/>'s remark for why. Disposing the client socket
+    /// abruptly, with no close handshake sent, is the closest this harness gets to that: the server-side
+    /// socket is left believing itself open until it actually tries to write to it.
+    /// </para>
+    /// <para>
+    /// Driven straight through <see cref="TerminalDataPlane.SendAsync"/> rather than through a pump, because
+    /// a pump adds nothing here — the point is entirely about the transport's own contract, and a session
+    /// layered on top would only leave it unclear whether a passing test proved the transport never threw or
+    /// merely that the frames never happened to need a live socket.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task SendAsync_DoesNotThrow_WhenTheAttachedRendererDiedWithoutClosing_AndAFreshAttachStillReceives()
+    {
+        Start();
+
+        var first = await ConnectAsync();
+        first.State.ShouldBe(WebSocketState.Open);
+        first.Dispose();
+
+        // Whether this particular send lands on the OS's send buffer before the peer's absence is noticed,
+        // or fails immediately, is not the point — either way it must not throw.
+        await Should.NotThrowAsync(async () =>
+            await plane.SendAsync(
+                TerminalFrame.Create((byte)TerminalServerOpcode.Output, SessionId, "before"u8.ToArray()),
+                TestContext.Current.CancellationToken));
+
+        using var second = await ConnectAsync();
+
+        await Should.NotThrowAsync(async () =>
+            await plane.SendAsync(
+                TerminalFrame.Create((byte)TerminalServerOpcode.Output, SessionId, "after"u8.ToArray()),
+                TestContext.Current.CancellationToken));
+
+        var output = await ReceiveAsync(second);
+        output.Opcode.ShouldBe((byte)TerminalServerOpcode.Output);
+        Encoding.UTF8.GetString(output.Payload).ShouldBe("after");
     }
 
     // ---- Frames ----