Make the terminal's right button copy and paste, and never open a menu

Selecting output and right-clicking it opened the WebView's own context menu with
Copy on it, which is correct behaviour for a web page and wrong for a terminal.
What a terminal's right button has meant since the Windows console got QuickEdit,
and since PuTTY shipped, is this: with something selected it copies and drops the
selection, with nothing selected it pastes. No menu, no second click.

The gesture can only live in the page. The selection is drawn into a canvas the
host never sees, and once a terminal has focus the host's window receives no input
events at all — the same reason the Ctrl+plus chords are handled there.

◆ THE CLIPBOARD IS THE HOST'S IN BOTH DIRECTIONS, AND THE READING IS WHY.

navigator.clipboard.readText() is gated behind the clipboard-read permission in
every Chromium, and an embedded WebView with no PermissionRequested handler
answers a permission request by putting a prompt on the screen. Trading a context
menu for a permission dialog is not this change. So the page asks and the host
answers, exactly as requestFontSizeStep does.

Writing does not go the other way either, although navigator.clipboard.writeText
would have worked from the page on Windows: a clipboard that is the host's in one
direction and the page's in the other is two mechanisms to keep working across
WebView2, WebKitGTK and Android's WebView. The host has owned a clipboard delegate
since the keychain grew its COPY buttons, so each App.axaml.cs grows a
ClipboardReader beside its ClipboardWriter and MainWindowViewModel takes it.

Three opcodes: client ClipboardCopy and ClipboardPaste, server Clipboard.

◆ Clipboard IS NOT Paste, AND THE DIFFERENCE IS THE BRACKETING.

Paste carries a snippet off the sidebar, and the page deliberately *types* a
single-line one rather than bracketing it, because bash marks a bracketed paste as
an active region and a snippet should read as though it had been typed. A clipboard
paste is the case those markers are for — readline uses them to decide it has been
pasted into — so Clipboard is always bracketed and never followed by an Enter. One
opcode for both would have needed a flag meaning "which of the two kinds of paste
is this", on a payload whose existing flag byte means something else.

Both client frames are dispatched before the session lookup, which is what keeps
the gesture working in a pane whose shell has just ended: squinting at the message
a remote left behind is exactly when somebody copies the line. Dispatch was past
MA0051's length once it grew a second such branch, so the frames no pump needs to
see moved into Announce() and the font-size chord went with them.

◆ CAPTURED AND STOPPED, RATHER THAN MERELY PREVENTED.

preventDefault alone suppresses the menu and still lets xterm's own contextmenu
handler run, which moves its helper textarea under the pointer at z-index 1000 and
leaves the selection sitting in it, selected. Both of those exist to serve the
native menu. Taking the event in the capture phase on the pane and stopping it
there is what keeps the gesture wholly the page's.

rightClickSelectsWord is now false, which on macOS is a change of default rather
than a restatement of it: xterm sets it true there alone, so a right-click would
have selected the word under the pointer and copied that instead of pasting —
wrong on one platform only, which is the worst way for it to be wrong.

The renderer harness grows the pane it has been inviting since
RendererReconnectionTests: a fake Terminal, the two addons, and a real UTF-8 codec,
because both directions run through the page's own TextEncoder and TextDecoder and
a Latin-1 stub would pass a test with an em dash in the selection. Its warning
about declaring names terminal.js declares now covers the half that does not
SyntaxError — a harness frame() was silently replaced by the page's frame() and
failed six tests on a type error a long way from the cause.

Manual checks 1.11 and 11.13 are what is left for a person: that a real contextmenu
event reaches a capture listener, that no native menu is drawn once it is
prevented, and that xterm's real paste() brackets. 11.13 also records what is
unverified — whether Android's WebView delivers a pointer's secondary click as a
contextmenu event at all. ClipboardReader is wired on that head regardless,
because the alternative is a head where the paste half silently does nothing and
nobody finds out; that head is not compiled here either, this machine having no
android workload.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -356,6 +356,82 @@ public sealed class TerminalDataPlaneTests : IAsyncDisposable
         steps[0].ShouldBe(-1);
     }
 
+    /// <remarks>
+    /// Dispatched before the session lookup for the reason the text-size chord is, and one of its own: a
+    /// clipboard is not a property of any terminal. The absence of a registered session is the point of the
+    /// test — copying the last thing a remote said before it hung up is an ordinary thing to want, and a
+    /// dead tab must not be the one place the gesture silently does nothing.
+    /// </remarks>
+    [Fact]
+    public async Task ACopy_IsHeardWithNoSessionRegistered()
+    {
+        Start();
+
+        using var socket = await ConnectAsync();
+
+        var copied = new List<string>();
+        plane.ClipboardCopyRequested += (_, e) => copied.Add(e.Text);
+
+        await SendAsync(
+            socket,
+            (byte)TerminalClientOpcode.ClipboardCopy,
+            Encoding.UTF8.GetBytes("error: nothing was written"));
+
+        await WaitUntilAsync(() => copied.Count > 0);
+
+        copied[0].ShouldBe("error: nothing was written");
+    }
+
+    /// <remarks>
+    /// An empty copy is dropped rather than forwarded. The clipboard is shared with everything else on the
+    /// machine, so "copy nothing" would be a gesture that quietly destroys whatever was on it — and the page
+    /// does not send one, which makes this a guard against a frame from something that is not the page.
+    /// </remarks>
+    [Fact]
+    public async Task AnEmptyCopy_IsDropped()
+    {
+        Start();
+
+        using var socket = await ConnectAsync();
+
+        var copies = 0;
+        plane.ClipboardCopyRequested += (_, _) => Interlocked.Increment(ref copies);
+
+        await SendAsync(socket, (byte)TerminalClientOpcode.ClipboardCopy, []);
+
+        // Followed by one that must be heard, so the assertion below is about ordering rather than about
+        // having waited long enough: the receive loop is sequential, so a copy arriving after this proves
+        // the empty one was seen and passed over.
+        var copied = new List<string>();
+        plane.ClipboardCopyRequested += (_, e) => copied.Add(e.Text);
+
+        await SendAsync(socket, (byte)TerminalClientOpcode.ClipboardCopy, "uptime"u8.ToArray());
+        await WaitUntilAsync(() => copied.Count > 0);
+
+        copies.ShouldBe(1, "only the second frame carried anything");
+    }
+
+    /// <remarks>
+    /// The session id is the whole payload of a paste request — it says which pane the pointer was over, and
+    /// therefore where the answer has to land. Heard with nothing registered for the reason a copy is.
+    /// </remarks>
+    [Fact]
+    public async Task APasteRequest_CarriesTheSessionItWasMadeIn()
+    {
+        Start();
+
+        using var socket = await ConnectAsync();
+
+        var asked = new List<uint>();
+        plane.ClipboardPasteRequested += (_, e) => asked.Add(e.SessionId);
+
+        await SendAsync(socket, (byte)TerminalClientOpcode.ClipboardPaste, [], sessionId: 4242);
+
+        await WaitUntilAsync(() => asked.Count > 0);
+
+        asked[0].ShouldBe(4242u);
+    }
+
     [Fact]
     public async Task AFrameForAnUnregisteredSession_IsIgnored()
     {