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]>

src/config/mod.rs

@@ -0,0 +1,6 @@
+//! Where cumulus keeps its files ([`paths`]), and every setting it has
+//! ([`settings`]). Reading config.toml, keys.toml and theme files comes with
+//! part 8b of the build.
+
+pub mod paths;
+pub mod settings;