fix(textarea): Up and Down land on the soft-wrapped row they target

A soft-wrapped row and the next row share a byte: the row's exclusive end
is the next row's start. Up or Down with a goal column past the target
row's width put the cursor on that byte, so it drew on the wrong row. Up
made no progress and Down skipped a row.

`row_with_end` tells a soft wrap from a hard break (byte-adjacent rows
are soft). `move_to_display_col_on_line` takes the row and its `RowEnd`
and clamps a soft row to its last grapheme, or to the start of an element
that closes the row. A hard-break row still reaches its newline.
`clamp_to_line` now cuts on graphemes, not chars, so a ZWJ cluster stays
whole.

Kigi differs from upstream's premise that elements wrap whole: with no
display override in the buffer, `wrapped_lines` uses plain textwrap and
an element can span rows. Snapping to such an element's start would
leave the target row (Down from byte 0 stuck at 0), so an element that
began on an earlier row keeps the boundary `display_col_to_buffer_pos`
chose, as before this change.

`wrapped_navigation_with_zwj_graphemes` asserted the bug: Up from the
end stayed on the last row (byte 22). It now expects row 1's cluster
start (byte 11), as upstream does.

Tests: upstream's four cases, the row-spanning element, and a seeded
property test (500 cases): each Up or Down moves exactly one row. The
property test fails with the clamp disabled.

Ported from upstream (grok-build ratatui-textarea/textarea.rs, 1.0.28).

crates/codegen/kigi-ratatui-textarea/src/textarea/soft_wrap_nav_tests.rs

@@ -0,0 +1,121 @@
+//! Up/Down across soft-wrapped rows: a soft row's exclusive end is the next row's start.
+
+use rand::prelude::*;
+use ratatui::layout::Rect;
+use ratatui::text::Line;
+
+use super::{ElementKind, TextArea};
+
+fn ta_with(text: &str) -> TextArea {
+    let mut t = TextArea::new();
+    t.insert_str(text);
+    t
+}
+
+fn cursor_row(t: &TextArea, width: u16) -> usize {
+    TextArea::wrapped_line_index_by_start(&t.wrapped_lines(width), t.cursor()).unwrap()
+}
+
+#[test]
+fn up_from_wide_row_lands_on_last_char_of_narrower_soft_row() {
+    let width = 8;
+    let mut t = ta_with("aaa bb ccccccc");
+    assert_eq!(vec![0..7, 7..14], *t.wrapped_lines(width));
+
+    t.move_cursor_up();
+    assert_eq!(6, t.cursor());
+    assert_eq!(0, cursor_row(&t, width));
+    assert_eq!(Some((6, 0)), t.cursor_pos(Rect::new(0, 0, width, 5)));
+
+    t.move_cursor_down();
+    assert_eq!(14, t.cursor());
+
+    t.move_cursor_up();
+    assert_eq!(6, t.cursor());
+    t.move_cursor_up();
+    assert_eq!(0, t.cursor());
+}
+
+#[test]
+fn up_onto_newline_terminated_row_lands_on_the_newline() {
+    let mut t = ta_with("ab\ncdef");
+    assert_eq!(vec![0..2, 3..7], *t.wrapped_lines(8));
+
+    t.move_cursor_up();
+    assert_eq!(2, t.cursor());
+    assert_eq!(0, cursor_row(&t, 8));
+}
+
+#[test]
+fn down_from_wide_row_lands_on_last_char_of_narrower_soft_row() {
+    let mut t = ta_with("ccccccc aaa bb ddddddd");
+    assert_eq!(vec![0..8, 8..15, 15..22], *t.wrapped_lines(8));
+    t.set_cursor(7);
+
+    t.move_cursor_down();
+    assert_eq!(14, t.cursor());
+    assert_eq!(1, cursor_row(&t, 8));
+
+    t.move_cursor_down();
+    assert_eq!(22, t.cursor());
+}
+
+#[test]
+fn up_onto_soft_row_closed_by_element_lands_before_the_element() {
+    let mut t = TextArea::new();
+    t.insert_element("xyz", ElementKind(0), Some(Line::from("[ELEM]")));
+    t.insert_str("longword");
+    assert_eq!(vec![0..3, 3..11], *t.wrapped_lines(8));
+
+    t.move_cursor_up();
+    assert_eq!(0, t.cursor());
+    assert_eq!(0, cursor_row(&t, 8));
+}
+
+#[test]
+fn down_through_element_spanning_rows_reaches_the_end() {
+    let mut t = TextArea::new();
+    t.insert_element("abcdefgh", ElementKind(0), None);
+    t.insert_str("z");
+    assert_eq!(vec![0..3, 3..6, 6..9], *t.wrapped_lines(3));
+    t.set_cursor(0);
+
+    t.move_cursor_down();
+    assert_eq!(8, t.cursor());
+}
+
+#[test]
+fn up_and_down_move_one_row_at_a_time() {
+    let mut rng = StdRng::seed_from_u64(7);
+    for _ in 0..500 {
+        let words = rng.random_range(1..12);
+        let text = (0..words)
+            .map(|_| "x".repeat(rng.random_range(1..9)))
+            .collect::<Vec<_>>()
+            .join(" ");
+        let width = rng.random_range(3..12);
+        let mut t = ta_with(&text);
+        let rows = t.wrapped_lines(width).len();
+        t.set_cursor(rng.random_range(0..=text.len()));
+
+        let mut row = cursor_row(&t, width);
+        while row + 1 < rows {
+            t.move_cursor_down();
+            assert_eq!(
+                row + 1,
+                cursor_row(&t, width),
+                "down in {text:?} at width {width}"
+            );
+            row += 1;
+        }
+        while row > 0 {
+            t.move_cursor_up();
+            assert_eq!(
+                row - 1,
+                cursor_row(&t, width),
+                "up in {text:?} at width {width}"
+            );
+            row -= 1;
+        }
+    }
+}