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]>

README.md

@@ -4,8 +4,8 @@ Brainstorm in the terminal: ideas as nodes on a 2D canvas, connected, folded
 and color-coded, across as many documents as you like. Vim keys first; the
 mouse works too. Runs on Windows and Linux.
 
-> **Status:** being built, one stage at a time. See [DESIGN.md](DESIGN.md) for
-> the plan; this README grows with the app.
+> **Status:** v1 is built: everything on [DESIGN.md](DESIGN.md)'s v1 list.
+> v1.1 (the board, a search across documents, tidying, export) comes next.
 
 ## Build and run
 
@@ -42,6 +42,12 @@ saves a second after each change, reloads a file another program changed
 A document open in another cumulus opens read-only. Each one reopens where you
 left it.
 
+A file's name follows its root's title: give the root a new title and the
+file is renamed (`ideas.cum` keeps its name), and the portals leading to it
+follow. Portals link documents: a portal shows its document's title, color
+and size, Enter goes through it, and each document knows the portals that
+lead to it (`« 2` on its root, `gu` goes up to them).
+
 ## Settings, keys and themes
 
 `:config` lists the three config files: `config.toml` (the settings),
@@ -82,10 +88,13 @@ The plan for the everyday keys (all of them are in DESIGN.md):
 | `K`               | the body in a card beside the node (`i` edits it there) |
 | `v` `V`           | select: the nodes between here and where you started (`V`: with their branches); then `d` `y` `>` `<` `M` or `<Space>c` act on all of them |
 | `M`               | move the branch with `hjkl` (`M` again: just the node) |
+| `C`               | connect: go to another node, Enter links them (`d` removes the line, `>` puts it under that node, `<Space>` `a` `l` `c` `s` `t` style it, Esc takes it all back) |
 | `/` `n` `N`       | search: titles, tags and bodies, fuzzily, with the note of each match beside it / the next match / the previous one |
 | `Ctrl-o` `Tab`    | back to where you jumped from / forward again (across documents) |
 | `ma` `'a` `mA`    | mark a node / go back to it / a capital mark: from any document |
 | `gd`              | follow a cross-link                                |
+| `<Space>p`        | a portal to another document (`:portal file`, or a new document's title) |
+| Enter or `gf` `gu` | through a portal / up to the portals that lead here |
 | `<Space>`         | the action menu: `c` color, `t` tags, `b` documents, `i` inbox, `vo` outline, `vn` note… |
 | `Ctrl-w v` `Ctrl-w s` | split the pane side by side / stacked (`Ctrl-w hjkl` to move between them) |
 | `gt` `gT`         | the next / previous document                       |