fix(io): update visual height of last line on append without trailing newline (closes #11)

When a file does not end with a newline, appending content extends the
last logical line's text and thus its visual height. The incremental
extend path in handle_file_appended only computed heights for newly
created logical lines, missing the old last line whose content changed.

Add VisualHeightIndex::replace_last_line_height() — an O(1) method that
rewrites the final prefix sum entry and total. Called before
extend_from_heights so the correct line is targeted.

Changes:
- progressive_reader.rs: add replace_last_line_height, pub with_params,
  7 VHI unit tests
- app.rs: save old_reader_line_count before update, recompute last old
  line height in extend path, 2 integration regression tests

crates/core/src/io/progressive_reader.rs

@@ -102,7 +102,7 @@ impl VisualHeightIndex {
         }
     }
 
-    fn with_params(mut self, json_format: bool, terminal_width: usize) -> Self {
+    pub fn with_params(mut self, json_format: bool, terminal_width: usize) -> Self {
         self.json_format = json_format;
         self.terminal_width = terminal_width;
         self
@@ -159,6 +159,31 @@ impl VisualHeightIndex {
             self.total_visual_rows += h as u64;
         }
     }
+
+    /// Replace the visual height of the last logical line. O(1).
+    ///
+    /// Must be called **before** `extend_from_heights` so that the last line
+    /// index still refers to the pre-extension line.
+    pub fn replace_last_line_height(&mut self, new_height: usize) {
+        let n = self.prefix_sums.len();
+        if n < 2 {
+            return;
+        }
+        let last_line = n - 2;
+        let old_height = self.prefix_sums[last_line + 1] - self.prefix_sums[last_line];
+        let new_height = new_height as u64;
+        if new_height == old_height {
+            return;
+        }
+        let delta = new_height.abs_diff(old_height);
+        if new_height > old_height {
+            self.prefix_sums[last_line + 1] += delta;
+            self.total_visual_rows += delta;
+        } else {
+            self.prefix_sums[last_line + 1] -= delta;
+            self.total_visual_rows -= delta;
+        }
+    }
 }
 
 // ─── VisualHeightRebuildResult ────────────────────────────────────────────────
@@ -1339,6 +1364,85 @@ mod tests {
         assert_eq!(idx.total_visual_rows(), 6);
     }
 
+    #[test]
+    fn test_replace_last_line_height_increase() {
+        let heights = [2, 3];
+        let mut idx = VisualHeightIndex::build(&heights);
+        assert_eq!(idx.total_visual_rows(), 5);
+        assert_eq!(idx.visual_height_of_line(1), 3);
+
+        idx.replace_last_line_height(7);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 7);
+        assert_eq!(idx.total_visual_rows(), 9);
+        assert_eq!(idx.cursor_to_first_visual_row(0), 0);
+        assert_eq!(idx.cursor_to_first_visual_row(1), 2);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_decrease() {
+        let heights = [2, 5];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(1);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 1);
+        assert_eq!(idx.total_visual_rows(), 3);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_same_is_noop() {
+        let heights = [2, 3];
+        let mut idx = VisualHeightIndex::build(&heights);
+        let total_before = idx.total_visual_rows();
+
+        idx.replace_last_line_height(3);
+
+        assert_eq!(idx.total_visual_rows(), total_before);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_then_extend() {
+        let heights = [2, 1];
+        let mut idx = VisualHeightIndex::build(&heights);
+        assert_eq!(idx.total_visual_rows(), 3);
+
+        idx.replace_last_line_height(4);
+        idx.extend_from_heights(&[3]);
+
+        assert_eq!(idx.line_count(), 3);
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 4);
+        assert_eq!(idx.visual_height_of_line(2), 3);
+        assert_eq!(idx.total_visual_rows(), 9);
+        assert_eq!(idx.cursor_to_first_visual_row(0), 0);
+        assert_eq!(idx.cursor_to_first_visual_row(1), 2);
+        assert_eq!(idx.cursor_to_first_visual_row(2), 6);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_single_line() {
+        let heights = [5];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(2);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.total_visual_rows(), 2);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_empty_index() {
+        let heights: [usize; 0] = [];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(5);
+
+        assert_eq!(idx.total_visual_rows(), 0);
+    }
+
     #[test]
     fn test_spawn_indexer_file_truncated_during_scan() {
         let mut content = Vec::new();