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]>

examples/kitchen.cum

@@ -40,4 +40,11 @@
 Empty it on Sundays.",
         ),
     ],
+    backlinks: [
+        (
+            from: "garden.cum",
+            uid: "7f3c9a1e52b04d18",
+            node: 10,
+        ),
+    ],
 )