Add visual mode, node registers, search and the jump list (stage 8c)

The rest of vim, on nodes. Visual mode takes in what lies between where
it started and the cursor: on the canvas, every node the rectangle from
one end's box to the other's touches; in the outline and the Note view,
the nodes in between in reading order. V takes whole branches, Ctrl+click
and Ctrl+drag pick with the mouse, and the operators take exactly what's
selected: d, y, > and <, M, and the pickers on every node at once.

Registers now hold text or nodes, so yy, dd and x keep what they take and
p, P and ]p paste it, layout and links included. "+ is the system
clipboard through arboard: nodes go there as Markdown in the user's
format (## headings, a --- tag list, the body), and come back whole while
the clipboard still holds them. clipboard = "unnamedplus" makes plain
yanks use it, as in vim.

. repeats the last change made with keys, with the text typed after it.
/ opens a Telescope-style search: fuzzy matches over titles, tags and
bodies, best first, with the note of the one you're on beside them; n
and N go on with it, and :find searches straight away. Each pane keeps a
jump list for Ctrl-o and Ctrl-i that crosses documents and reopens closed
ones. Capital marks lead into any document and, like the small ones, are
kept in memory.ron. gd follows a cross-link, and :marks, :jumps and
:registers list what they hold.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/clipboard.rs

@@ -0,0 +1,62 @@
+//! The system clipboard, behind the `"+` register (and behind every yank
+//! and paste with `clipboard = "unnamedplus"`). It's opened the first time
+//! it's used, and kept open: on Linux, text copied there lasts as long as
+//! cumulus runs, or until a clipboard manager takes it over.
+//!
+//! The tests keep their clipboard in memory, so they never touch yours.
+
+/// The system clipboard.
+#[derive(Default)]
+pub struct Clipboard {
+    #[cfg(not(test))]
+    system: Option<arboard::Clipboard>,
+    #[cfg(test)]
+    text: Option<String>,
+}
+
+impl std::fmt::Debug for Clipboard {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("Clipboard")
+    }
+}
+
+#[cfg(not(test))]
+impl Clipboard {
+    /// The text on the clipboard.
+    pub fn read(&mut self) -> Result<String, String> {
+        self.open()?.get_text().map_err(problem)
+    }
+
+    /// Puts text on the clipboard.
+    pub fn write(&mut self, text: &str) -> Result<(), String> {
+        self.open()?.set_text(text.to_string()).map_err(problem)
+    }
+
+    fn open(&mut self) -> Result<&mut arboard::Clipboard, String> {
+        if self.system.is_none() {
+            self.system = Some(arboard::Clipboard::new().map_err(problem)?);
+        }
+        self.system.as_mut().ok_or_else(|| "the clipboard isn't available".to_string())
+    }
+}
+
+/// What went wrong with the clipboard, in words.
+#[cfg(not(test))]
+fn problem(error: arboard::Error) -> String {
+    match error {
+        arboard::Error::ContentNotAvailable => "the clipboard holds no text".to_string(),
+        error => format!("the clipboard isn't available: {error}"),
+    }
+}
+
+#[cfg(test)]
+impl Clipboard {
+    pub fn read(&mut self) -> Result<String, String> {
+        self.text.clone().ok_or_else(|| "the clipboard holds no text".to_string())
+    }
+
+    pub fn write(&mut self, text: &str) -> Result<(), String> {
+        self.text = Some(text.to_string());
+        Ok(())
+    }
+}