feat(textarea): add atomic range editing support (#19)

- add public AtomicRange API with validation and diagnostic errors
- make cursor movement, deletion, insertion, selection, undo/redo, and set_lines atom-aware
- clear caller-derived atomic ranges after successful content mutations
- add focused atomic range tests plus serde coverage
- document atomic ranges and add a caller-owned parsing example

examples/atomic_ranges.rs

@@ -0,0 +1,67 @@
+use ratatui::style::{Color, Style};
+use tui_textarea::{AtomicDeleteDirection, AtomicRange, CursorMove, TextArea};
+
+fn placeholder_ranges(row: usize, line: &str) -> Vec<AtomicRange> {
+    let mut ranges = Vec::new();
+    let mut offset = 0;
+
+    while let Some(start_delta) = line[offset..].find("[[") {
+        let start_byte = offset + start_delta;
+        let Some(end_delta) = line[start_byte..].find("]]") else {
+            break;
+        };
+        let end_byte = start_byte + end_delta + 2;
+        ranges.push(AtomicRange {
+            row,
+            start_col: line[..start_byte].chars().count(),
+            end_col: line[..end_byte].chars().count(),
+        });
+        offset = end_byte;
+    }
+
+    ranges
+}
+
+fn byte_offset(line: &str, col: usize) -> usize {
+    line.char_indices()
+        .nth(col)
+        .map(|(offset, _)| offset)
+        .unwrap_or(line.len())
+}
+
+fn refresh_atoms(textarea: &mut TextArea<'_>) {
+    let ranges: Vec<_> = textarea
+        .lines()
+        .iter()
+        .enumerate()
+        .flat_map(|(row, line)| placeholder_ranges(row, line))
+        .collect();
+    let highlights: Vec<_> = ranges
+        .iter()
+        .map(|range| {
+            let line = &textarea.lines()[range.row];
+            (
+                (
+                    (range.row, byte_offset(line, range.start_col)),
+                    (range.row, byte_offset(line, range.end_col)),
+                ),
+                Style::default().fg(Color::Yellow),
+            )
+        })
+        .collect();
+
+    textarea.set_atomic_ranges(ranges);
+    textarea.clear_custom_highlight();
+    for (range, style) in highlights {
+        textarea.custom_highlight(range, style, 10);
+    }
+}
+
+fn main() {
+    let mut textarea = TextArea::from(["See [[image:cat.png]] before sending."]);
+    refresh_atoms(&mut textarea);
+
+    textarea.move_cursor(CursorMove::Jump(0, 4));
+    textarea.delete_atomic_range_at_cursor(AtomicDeleteDirection::Forward);
+    refresh_atoms(&mut textarea);
+}