Add connect mode, portals, backlinks and renames (stage 8d, v1)

Part 8d, the last of stage 8, and with it everything on DESIGN.md's v1
list.

Connect mode (C): the cursor goes to another node, a dashed arrow in the
visual color follows it, and Enter links the two; then the cursor goes
back to where C was typed (the user's pick). d removes the line between
them (a branch is cut, and the child comes loose), > puts the node under
the other one, and <Space> a, l, c, s and t style the line as you watch,
making it when it isn't there yet. Everything connect mode changes is one
undo group, and Esc takes it back with History::cancel, which leaves
nothing to redo. From visual mode every selected node connects, and the
line pickers go back to connect mode when they close.

Portals show their document live, from a cache of what each document
file holds (open documents from their buffers, the others from their
files, looked at every second): the root's title, its color and how many
nodes it has, or ? when the document isn't there. Enter or gf goes
through, as a jump, and the pane's breadcrumb starts with the documents
it came through. <Space>p types :portal (the user's pick): a path as :e
takes it, or the title of a new document. Backlinks are put right
whenever a document is opened or saved; the root shows how many, the
Note view lists them, gu goes up to them (the user's pick) and
:backlinks lists them.

File names follow the root's title: when it changed, the next save
renames the file (-2 when the name is taken; a folder's default document
keeps its name), and the portals pointing at it, its backlinks in the
documents it leads to, the recent documents, the capital marks and the
jump lists follow. A portal whose file was moved by hand finds its
document in that folder by uid. Portals in a register keep full paths,
so a pasted portal leads to the same document from any folder.

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

src/app/action.rs

@@ -7,6 +7,7 @@
 
 use super::documents::BufferId;
 use super::jumps::Place;
+use super::mode::Connect;
 use super::panes::Arrange;
 use super::popup::{MenuFor, PickKind};
 use super::repeat::Look;
@@ -96,6 +97,9 @@ pub enum DocAction {
     SetMark(char),
     /// Go to a mark; `'` and `` ` `` go back to before the last jump.
     JumpToMark(char),
+    /// Take back the group's undo step, if it's the newest, leaving nothing
+    /// to redo: Esc in connect mode.
+    Cancel(Group),
 }
 
 /// What happens around the documents.
@@ -124,6 +128,21 @@ pub enum AppAction {
     EnterMoveMode(Vec<NodeId>),
     /// In move mode: the whole branch, or just the node.
     ToggleWhole,
+    /// Connect mode (`C`): from the cursor's node, or from these (a
+    /// selection's).
+    EnterConnectMode(Vec<NodeId>),
+    /// Back to connect mode, from a picker it opened.
+    ResumeConnect(Connect),
+    /// Enter or `gf` on a portal: through it, to its document's root.
+    FollowPortal(NodeId),
+    /// `:portal`: a portal under the cursor's node, to the document at the
+    /// path typed (a new document when nothing's there).
+    NewPortal(String),
+    /// `gu`: up to the portals that lead to this document; `list`
+    /// (`:backlinks`) lists them even when there's just one.
+    GoUp {
+        list: bool,
+    },
     /// Back to normal mode.
     LeaveMode,
     OpenCommandLine,