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]>

Cargo.toml

@@ -24,6 +24,9 @@ unicode-width = "0.2"
 # Where one character ends and the next begins, so the cursor steps over an
 # accented letter or an emoji in one go.
 unicode-segmentation = "1"
+# Where each OS keeps settings and app data: %APPDATA% on Windows,
+# ~/.config and ~/.local/share on Linux.
+directories = "6"
 
 [lints.rust]
 unsafe_code = "forbid"
@@ -37,8 +40,3 @@ all = { level = "warn", priority = -1 }
 lto = true
 codegen-units = 1
 strip = true
-
-# The canvas example is the build's checkpoint: `cargo test` runs its tests too.
-[[example]]
-name = "canvas"
-test = true