fix(diff): reconcile the diff-overlay work with the host-aware git refactor

The rebase onto main lands this change on top of the remote-workspace work
(#235, #242), which moved the git helpers into `tty7-core` and made every read
take the pane's `Host`. Adapting rather than papering over:

- `sidebar_diff_preview` moves to the core `Config`, where the struct now lives.
- The diff and untracked reads go through `git_status::git(host, cwd, ..)`.
- The shared probe, its in-flight set, and `install_diff_snapshot` key on
  (`HostId`, `PathBuf`) — the same path on two machines is two work trees — as
  does the Changes panel's `diff_pending`.
- `diff_click_cwd` became generic over what identifies a repo, so the setting
  gate did not need to learn about hosts.
- Main's newer card rounding reads `truncated`, which is an `Option` here now.

The streaming diff read is deliberately absent at this commit: `Host::git` is
buffered, so it is restored on top of a streaming host API in the next one.

Refs #239.

src/terminal/git_diff.rs

@@ -295,10 +295,9 @@ pub fn probe(host: &dyn Host, cwd: &Path) -> Option<DiffSnapshot> {
 /// and the first hunk (modes, index, similarity) are simply skipped, so a git
 /// version printing extra headers degrades to "fewer facts", never a panic.
 ///
-/// The whole-string form the tests drive the parser through; [`probe`] streams
-/// the same [`DiffParser`] line by line off git's stdout instead, so no caller
-/// in the app ever holds the full diff as one `String`.
-#[cfg(test)]
+/// The whole-string form: [`probe`] hands it the output of one `git diff`
+/// invocation. [`DiffParser`] underneath is incremental, so a caller that can
+/// feed lines as they arrive does not have to hold the whole diff at once.
 pub fn parse_unified(out: &str) -> Vec<FileDiff> {
     let mut parser = DiffParser::default();
     for line in out.lines() {

src/ui/diff_overlay.rs

@@ -184,7 +184,7 @@ impl Tty7App {
         // probe still flies below, so the seeded view is never the last word.
         // Read here, before the `&mut` borrow of the tab.
         let seed = match (&self.right_panel.diff_cwd, &self.right_panel.diff) {
-            (Some(panel_cwd), Some(Some(snap))) if *panel_cwd == cwd => {
+            (Some(panel_key), Some(Some(snap))) if *panel_key == (host, cwd.clone()) => {
                 DiffLoad::Ready(Arc::clone(snap))
             }
             _ => DiffLoad::Loading,
@@ -740,7 +740,7 @@ impl Tty7App {
         // question: a binary file or a pure rename has no hunks and is not
         // truncated, so its body is empty and the header *is* the card. A
         // truncated file with no parsable hunks still renders the notice.
-        let has_body = expanded && (!file.hunks.is_empty() || file.truncated);
+        let has_body = expanded && (!file.hunks.is_empty() || file.truncated.is_some());
 
         // The header paints a solid band flush into the card's corners, and the
         // card's `overflow_hidden` cannot round it — that clip is a square,
@@ -858,7 +858,7 @@ impl Tty7App {
                 .iter()
                 .map(|hunk| (hunk, split_hunk(&hunk.lines)))
                 .collect();
-            let closing_row = if file.truncated {
+            let closing_row = if file.truncated.is_some() {
                 None
             } else {
                 hunks

src/ui/right_panel.rs

@@ -1186,7 +1186,7 @@ impl Tty7App {
         if self.right_panel.diff_cwd.as_ref() != Some(&key) {
             self.right_panel.diff_cwd = Some(key);
             self.right_panel.diff = None;
-            self.spawn_right_panel_diff(cwd.clone(), cx);
+            self.spawn_right_panel_diff(host.clone(), cwd.clone(), cx);
         } else if self.right_panel.diff.is_none() && self.right_panel.diff_pending.is_none() {
             // Nothing cached and nothing in flight: a probe for a previous cwd
             // landed after we had already moved on and dropped its result, so

src/ui/tab_sidebar.rs

@@ -1338,9 +1338,9 @@ mod tests {
     #[test]
     fn diff_click_target_needs_a_repo_either_way() {
         let mut cfg = Config::default();
-        assert_eq!(diff_click_cwd(&cfg, None), None);
+        assert_eq!(diff_click_cwd::<PathBuf>(&cfg, None), None);
         cfg.sidebar_diff_preview = false;
-        assert_eq!(diff_click_cwd(&cfg, None), None);
+        assert_eq!(diff_click_cwd::<PathBuf>(&cfg, None), None);
     }
 
     /// Groups appear in first-appearance order with Scratch pinned last, and