Run cumulus for real: the app core and its files (stage 8a)

The playground becomes app::App, and `cumulus` runs it: the main loop
with terminal setup and a panic hook, and the startup rules (./ideas.cum,
else ~/ideas.cum, made the first time; files and folders by name;
`cumulus new`; --paths).

The documents' files:
- autosave a second after the latest change (`:set noautosave` for
  vim-style :w), [+] in the document bar for unsaved changes; quitting
  saves first; without autosave :q refuses, :q! drops, ZZ saves first;
- a file changed outside cumulus is read again as an undo step; with
  unsaved changes here too, cumulus asks which to keep; a file with a
  mistake in it holds autosave until it's fixed or :w saves over it; a
  deleted one waits for :w; nothing is reloaded under an edit;
- the day's first save backs the file up to the data folder (30 days);
- a document open in another cumulus is read-only ([RO]) until that one
  lets go: OS file locks in the data folder, so a crash leaves none;
- where you left off (cursor, zoom, scroll), the recent documents (now
  in the documents picker) and the command history, in memory.ron;
- :e (alone: read it again; ! drops changes), :new, :bd, :w, :wa, :x,
  :qa, <Space>i for :in, and :set for every setting. glyphs=ascii now
  reaches the frames and bars too, not just the canvas and outline.

Stage 8 comes in four parts with a commit each (the user's choice); 8b
is the settings files. The example is now a playground running the app
on copies of the sample documents in a temporary folder, and the
end-to-end tests moved to src/app/tests.rs, with tests for the files.

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

README.md

@@ -26,6 +26,22 @@ a folder on your `PATH`.
   `target/x86_64-unknown-linux-musl/release/cumulus` and runs on any
   distribution.
 
+## Starting it
+
+```sh
+cumulus                      # ./ideas.cum, or ~/ideas.cum (made the first time)
+cumulus garden.cum           # that document (a new one if it isn't there yet)
+cumulus notes/               # that folder's ideas.cum
+cumulus new "Garden plans"   # makes ./garden-plans.cum (cumulus new: ./ideas.cum)
+cumulus --paths              # where cumulus keeps its settings and data
+```
+
+Documents are plain text files (`.cum`) that live wherever you like. cumulus
+saves a second after each change, reloads a file another program changed
+(`u` undoes that), and keeps a copy of each document a day in its data folder.
+A document open in another cumulus opens read-only. Each one reopens where you
+left it.
+
 ## Keys
 
 The plan for the everyday keys (all of them are in DESIGN.md):
@@ -43,10 +59,12 @@ The plan for the everyday keys (all of them are in DESIGN.md):
 | `+` `-`           | zoom in / out                                      |
 | `K`               | the body in a card beside the node (`i` edits it there) |
 | `C` `M` `v`       | connect, move, select                              |
-| `<Space>`         | the action menu: `c` color, `t` tags, `vo` outline, `vn` note… |
+| `<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                       |
 | `:` `?`           | command line, help (search it with `/`, Enter runs the key or command) |
+| `:e` `:new` `:bd` | open a document / make one / close it              |
+| `:w` `:q` `ZZ`    | save / close the pane (the last one quits) / save and close |
 
 After the first key of a sequence (`g`, `z`, `d`, `<Space>`…), a popup lists
 what can follow. The right mouse button opens a menu of what you can do with
@@ -61,10 +79,13 @@ cargo lint    # clippy, treating warnings as errors
 cargo fmt     # format the code
 ```
 
-While the app is being built, the examples try out each part:
+To try things without touching your own documents:
 
 ```sh
-cargo run --example canvas                         # everything so far: the views, splits, popups, editing
-cargo test show_canvas -- --ignored --nocapture    # print it at each zoom, as text
-cargo test --example canvas                        # type key sequences into it, and check what they do
+cargo run --example canvas                         # cumulus on copies of two sample documents
+cargo test show_canvas -- --ignored --nocapture    # print the sample at each zoom, as text
 ```
+
+The end-to-end tests (`src/app/tests.rs`) type keys and click on a test
+terminal, with the sample documents copied to a temporary folder, and check
+the screen, the documents and the files.