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/storage/stamp.rs

@@ -0,0 +1,47 @@
+//! Noticing that something else changed a file: cumulus remembers each
+//! document file's [`Stamp`] when it reads or writes it, and looks again
+//! every second. A different stamp means another program (an editor, a
+//! sync tool, another cumulus) wrote the file, and cumulus reloads it.
+//! Looking costs one quick question to the file system per document, so
+//! there's no need for a file-watching library.
+
+use std::fs;
+use std::io;
+use std::path::Path;
+use std::time::SystemTime;
+
+/// When a file was last written, and how long it is.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Stamp {
+    modified: SystemTime,
+    len: u64,
+}
+
+impl Stamp {
+    /// The stamp of the file at `path`, or `None` when there's no file there.
+    pub fn of(path: &Path) -> io::Result<Option<Stamp>> {
+        match fs::metadata(path) {
+            Ok(metadata) => Ok(Some(Stamp { modified: metadata.modified()?, len: metadata.len() })),
+            Err(error) if error.kind() == io::ErrorKind::NotFound => Ok(None),
+            Err(error) => Err(error),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::storage::test_support::TempDir;
+
+    #[test]
+    fn a_write_changes_the_stamp() {
+        let folder = TempDir::new();
+        let path = folder.path().join("garden.cum");
+        assert_eq!(Stamp::of(&path).unwrap(), None);
+        fs::write(&path, "one").unwrap();
+        let first = Stamp::of(&path).unwrap();
+        assert_eq!(Stamp::of(&path).unwrap(), first, "nothing changed");
+        fs::write(&path, "three").unwrap();
+        assert_ne!(Stamp::of(&path).unwrap(), first);
+    }
+}