R6.6 round 1 fix: no Git says to install ElyTree; Linux keeps Mark's Git

git::own returns git::NoGit when neither the app nor ElyTree has Git;
ElyMark's Git item then reads 'needs ElyTree'. Linux AppImages stand
alone, so ElyMark keeps its Git there (packages share the AppDir).
The bundle fixture now has ElyTree's executable folder.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/ely-platform/src/git.rs

@@ -32,7 +32,7 @@ pub fn locate() -> io::Result<(PathBuf, bool)> {
         Some(serde_json::Value::String(path)) => return Ok((PathBuf::from(path), false)),
         Some(other) => return Err(io::Error::new(io::ErrorKind::InvalidData, format!("settings: git cannot be {other}"))),
     }
-    Ok((own(&std::env::current_exe()?), true))
+    Ok((own(&std::env::current_exe()?)?, true))
 }
 
 /// `git version 2.56.0…` as (2, 56); None for anything else.
@@ -64,8 +64,20 @@ pub fn system_version(path: &Path) -> io::Result<String> {
     Ok(version)
 }
 
+/// No Git beside the app nor beside ElyTree.
+#[derive(Debug)]
+pub struct NoGit;
+
+impl std::fmt::Display for NoGit {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        f.write_str("Ely's Git is missing: ElyTree carries it")
+    }
+}
+
+impl std::error::Error for NoGit {}
+
 /// The app at `exe`'s own Git, else ElyTree's, else built.
-fn own(exe: &Path) -> PathBuf {
+fn own(exe: &Path) -> io::Result<PathBuf> {
     // ElyTree carries Git; the other apps use its copy.
     let trees = crate::apps::beside(exe, AppId::Tree).into_iter().chain(crate::apps::registered(AppId::Tree));
     let built = Path::new(env!("CARGO_MANIFEST_DIR")).join("../../engines/git").join(GIT);
@@ -74,8 +86,9 @@ fn own(exe: &Path) -> PathBuf {
         .into_iter()
         .chain(trees.filter_map(|tree| tree.parent().map(Path::to_path_buf)))
         .flat_map(|dir| [dir.join("git").join(GIT), dir.join("../Resources/git").join(GIT)])
+        .chain([built])
         .find(|path| path.is_file())
-        .unwrap_or(built)
+        .ok_or_else(|| io::Error::new(io::ErrorKind::NotFound, NoGit))
 }
 
 /// Git at `git` in `dir`, as Ely runs it.
@@ -125,23 +138,26 @@ mod tests {
         let mark = apps.path().join("ElyMark.app/Contents/MacOS");
         let git = apps.path().join("ElyTree.app/Contents/Resources/git").join(GIT);
         std::fs::create_dir_all(&mark).expect("ElyMark");
+        std::fs::create_dir_all(apps.path().join("ElyTree.app/Contents/MacOS")).expect("ElyTree");
         let exe = mark.join("ely-mark");
-        assert!(own(&exe).ends_with(std::path::Path::new("engines/git").join(GIT)));
+        let built = own(&exe).expect("the built Git");
+        assert!(built.ends_with(std::path::Path::new("engines/git").join(GIT)));
         std::fs::create_dir_all(git.parent().expect("a folder")).expect("ElyTree's Git");
         std::fs::write(&git, "").expect("git");
-        assert_eq!(std::fs::canonicalize(own(&exe)).expect("found"), std::fs::canonicalize(&git).expect("git"));
+        let found = own(&exe).expect("ElyTree's Git");
+        assert_eq!(std::fs::canonicalize(found).expect("found"), std::fs::canonicalize(&git).expect("git"));
     }
 
     #[test]
-    fn windows_and_linux_installs_use_elytrees_git() {
-        for (mark, tree) in [("Ely Office/ElyMark", "Ely Office/ElyTree"), ("ely-office/ely-mark/bin", "ely-office/ely-tree/bin")] {
+    fn a_windows_install_uses_elytrees_git() {
+        for (mark, tree) in [("Ely Office/ElyMark", "Ely Office/ElyTree")] {
             let root = tempfile::tempdir().expect("a temp dir");
             let exe = root.path().join(mark).join("ely-mark");
             let git = root.path().join(tree).join("git").join(GIT);
             std::fs::create_dir_all(exe.parent().expect("a folder")).expect("ElyMark");
             std::fs::create_dir_all(git.parent().expect("a folder")).expect("ElyTree's Git");
             std::fs::write(&git, "").expect("git");
-            let found = std::fs::canonicalize(own(&exe)).expect("found");
+            let found = std::fs::canonicalize(own(&exe).expect("ElyTree's Git")).expect("found");
             assert_eq!(found, std::fs::canonicalize(&git).expect("git"), "{mark}");
         }
     }