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>

src/DodoSSH.Client.Terminal/TerminalFrame.cs

@@ -95,6 +95,31 @@ public enum TerminalServerOpcode : byte
     /// </para>
     /// </remarks>
     FontSize = 7,
+
+    /// <summary>
+    /// The system clipboard's text, for this session's pane to paste. Payload is UTF-8.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Sent in answer to <see cref="TerminalClientOpcode.ClipboardPaste"/>, which is the only thing that
+    /// asks for it: a right-click over a pane with nothing selected. The host reads the clipboard because
+    /// the page cannot — see that opcode for why not.
+    /// </para>
+    /// <para>
+    /// <b>Not <see cref="Paste"/>, and the difference is the bracketing.</b> That opcode carries a snippet
+    /// off the sidebar, and the page deliberately <em>types</em> 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 this one is always bracketed. One opcode for both would need a flag
+    /// meaning "which of the two kinds of paste is this", which is the same distinction spelled less
+    /// clearly, on a payload that already has a flag byte meaning something else.
+    /// </para>
+    /// <para>
+    /// No flag byte, therefore, and no Enter: a clipboard paste inserts and stops. Pressing Enter for
+    /// somebody is what the sidebar's RUN does, and it is a thing they asked for by name.
+    /// </para>
+    /// </remarks>
+    Clipboard = 8,
 }
 
 /// <summary>Frames the renderer sends to the host.</summary>
@@ -131,6 +156,52 @@ public enum TerminalClientOpcode : byte
     /// </para>
     /// </remarks>
     FontSizeStep = 4,
+
+    /// <summary>
+    /// Text to put on the system clipboard. Payload is UTF-8.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// A right-click over a pane with something selected. What gets copied is the selection, and only the
+    /// page knows what that is — xterm draws it into a canvas and the host never sees a glyph of it — so it
+    /// is the text that travels here rather than a request for it.
+    /// </para>
+    /// <para>
+    /// <b>Through the host rather than through the page's own <c>navigator.clipboard</c>.</b> Writing the
+    /// clipboard from the page would work on Windows, where the renderer is Blink and
+    /// <c>clipboard-write</c> is granted to a focused document without asking. It is the <em>reading</em>
+    /// that cannot be done that way — see <see cref="ClipboardPaste"/> — and 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; this is that one.
+    /// </para>
+    /// <para>
+    /// The session id is the pane the selection came from. Nothing routes on it — a clipboard is not a
+    /// property of one terminal — and it is carried because every frame carries one.
+    /// </para>
+    /// </remarks>
+    ClipboardCopy = 5,
+
+    /// <summary>
+    /// The page asking for the clipboard's text, to paste into this session. No payload.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// A right-click over a pane with nothing selected. The answer arrives as
+    /// <see cref="TerminalServerOpcode.Clipboard"/>, addressed to the session id this frame carried, so the
+    /// text lands in the pane the pointer was over rather than in whichever one the host believes is
+    /// selected. They agree in every arrangement that exists today, because only the active pane is
+    /// visible — but the page is the one that knows, so the page is the one that says.
+    /// </para>
+    /// <para>
+    /// <b>A request rather than the page reading the clipboard itself, and this is the half with no
+    /// alternative.</b> <c>navigator.clipboard.readText()</c> is gated behind the <c>clipboard-read</c>
+    /// permission in every Chromium, and an embedded WebView answers a permission request it has no handler
+    /// for by putting a prompt on the screen. Trading a context menu for a permission dialog is not the
+    /// change this was.
+    /// </para>
+    /// </remarks>
+    ClipboardPaste = 6,
 }
 
 /// <summary>