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/command/parse.rs

@@ -72,8 +72,9 @@ pub enum SetArg {
 /// What a command line asks for, with its arguments checked and typed.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Call {
+    /// Open a document; no path reads the focused one again from its file.
     Edit {
-        path: String,
+        path: Option<String>,
         force: bool,
     },
     New(Option<Title>),
@@ -240,7 +241,7 @@ fn build(spec: &Spec, bang: bool, args: &[Option<&str>], sources: &Sources) -> R
     let second = args.get(1).copied().flatten();
     let text = first.unwrap_or_default();
     Ok(match spec.id {
-        Id::Edit => Call::Edit { path: text.to_string(), force: bang },
+        Id::Edit => Call::Edit { path: first.map(str::to_string), force: bang },
         Id::New => Call::New(first.map(title).transpose()?),
         Id::Write => Call::Write,
         Id::WriteAll => Call::WriteAll,
@@ -479,7 +480,7 @@ mod tests {
         assert_eq!(call("x"), Ok(Call::WriteQuit));
         assert_eq!(
             call("e notes/garden.cum"),
-            Ok(Call::Edit { path: "notes/garden.cum".into(), force: false })
+            Ok(Call::Edit { path: Some("notes/garden.cum".into()), force: false })
         );
         assert_eq!(call("vs outline"), Ok(Call::VSplit(Some(ViewKind::Outline))));
         assert_eq!(call("in buy a rain barrel"), Ok(Call::Capture("buy a rain barrel".into())));
@@ -517,7 +518,7 @@ mod tests {
         assert_eq!(message(""), "type a command, like :help");
         assert_eq!(message("folx open"), "there's no command :folx; did you mean :fold?");
         assert_eq!(message("w!"), ":write doesn't take a !");
-        assert_eq!(message("e"), ":e[dit][!] <file>: the file is missing");
+        assert_eq!(message("b"), ":b[uffer] <document>: the document is missing");
         assert_eq!(message("fold up"), "\"up\" isn't a way to fold: use open, auto, closed");
         assert_eq!(message("fold open now"), ":fo[ld] <open|auto|closed>: \"now\" is more than it takes");
         assert_eq!(message("color corla"), "\"corla\" isn't a color: use red, orange, teal");