fix(tui): preserve old state on load_file failure (issue #32)

load_file previously cleared file_watcher before attempting to open the
new file. If open failed, the watcher was lost while loading_state and
file_path still pointed to the old file, breaking change detection.

Refactor to build-then-swap pattern: all fallible work runs on local
variables first, then self is updated only on success. On failure, the
old file's watcher, loading_state and file_path remain intact.

Update test to verify all three fields are preserved on failure.

Closes #32

crates/tui/src/app.rs

@@ -150,13 +150,13 @@ impl App {
     }
 
     pub fn load_file(&mut self, path: &str) -> anyhow::Result<()> {
-        // Cancel any existing background indexer by dropping the old state
-        self.file_watcher = None;
-
+        // ── Phase 1: Pure computation, no mutation of self ──────────
+        // If any step fails and returns ?, self remains completely untouched,
+        // preserving the old file's watcher, loading_state, and file_path.
         let mut pfr = ProgressiveFileReader::open(Path::new(path))
             .map_err(|e| anyhow::anyhow!("{e}"))?;
 
-        if pfr.is_sampling() {
+        let new_loading_state = if pfr.is_sampling() {
             // Cache miss: spawn background indexer
             let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
             let generation = pfr.generation();
@@ -175,19 +175,27 @@ impl App {
             ).map_err(|e| anyhow::anyhow!("{e}"))?;
 
             let estimated = pfr.line_count() as u64;
-            self.loading_state = AppLoadingState::Loading {
+            AppLoadingState::Loading {
                 reader: pfr,
                 estimated_lines: estimated,
                 progress_percent: 0.0,
-            };
+            }
         } else {
             // Cache hit: Ready state
-            self.loading_state = AppLoadingState::Ready { reader: pfr };
-        }
+            AppLoadingState::Ready { reader: pfr }
+        };
 
-        self.file_watcher = FileWatcher::watch(Path::new(path)).ok();
+        let new_watcher = FileWatcher::watch(Path::new(path)).ok();
 
+        // ── Phase 2: Commit — swap self to new state ───────────────
+        // SAFETY: Do NOT add any fallible operations (with ?) below this point.
+        // The old file_watcher and loading_state are dropped here, cancelling
+        // any background indexer for the previous file.
+        self.file_watcher = new_watcher;
+        self.loading_state = new_loading_state;
         self.file_path = Some(path.to_string());
+
+        // Reset UI state for the new file
         self.cursor_line = 0;
         self.v_offset = 0;
         self.v_sub_offset = 0;
@@ -2351,17 +2359,28 @@ plain text line
     }
 
     #[test]
-    fn test_file_watcher_stopped_on_nonexistent_file() {
+    fn test_file_watcher_preserved_on_nonexistent_file() {
         let path = make_temp_file("data\n");
         let result = std::panic::catch_unwind(|| {
             let mut app = App::new();
             app.load_file(path.to_str().unwrap()).unwrap();
             assert!(app.file_watcher.is_some());
 
-            let _ = app.load_file("/tmp/no_such_file_log_viewer_test_xyz_999");
+            let result = app.load_file("/tmp/no_such_file_log_viewer_test_xyz_999");
+            assert!(result.is_err());
+
             assert!(
-                app.file_watcher.is_none(),
-                "file_watcher should be None after failed load_file"
+                app.file_watcher.is_some(),
+                "file_watcher should be preserved after failed load_file"
+            );
+            assert_eq!(
+                app.file_path.as_deref(),
+                Some(path.to_str().unwrap()),
+                "file_path should be preserved after failed load_file"
+            );
+            assert!(
+                app.is_loaded(),
+                "loading_state should remain Ready after failed load_file"
             );
         });
         cleanup(&path);