feat(clipboard): PASTESPEC pastes in a chosen format

PASTESPEC offers the formats on the clipboard: Objects and Block for
objects copied in the application (the PASTECLIP and PASTEBLOCK paths),
and Text for the system clipboard's text, placed as multi-line text in
the current text style with its lines as paragraphs.

Refs #645

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -2360,6 +2360,8 @@ pub enum Message {
     PasteShortcut,
     /// Which text input held focus when `PasteShortcut` arrived.
     PasteShortcutResolved(PasteFocus),
+    /// Completion of the system text clipboard read PASTESPEC starts.
+    PasteSpecial(Option<String>),
     /// Completion of a system text clipboard read requested by PASTECLIP.
     SystemClipboardPaste(SystemClipboardText),
     /// Web: completion of the clipboard-image read the paste fallback starts