Insert Reference panel: one Attach, labelled underlay tools, slide-out with Edit Reference and xref fading

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

src/app/mod.rs

@@ -3814,6 +3814,11 @@ pub enum Message {
     XAttachPickResult(Result<std::path::PathBuf, String>),
     /// ATTACH: pick a drawing, image or PDF to reference.
     AttachPick,
+    /// The Reference slide-out's xref fading: amount dragged, drag done,
+    /// switch.
+    XrefFadeSlide(u8),
+    XrefFadeCommit,
+    XrefFadeToggle,
     /// Result of the ATTACH file picker.
     AttachPickResult(Result<std::path::PathBuf, String>),
     /// An edit in the Attach External Reference dialog.