Avoid cloning Rows during Galley::concat (#7649)

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Moves `ends_with_newline` into `PlacedRow` to avoid clones during
layout.
I don't think there was a rationale stronger than "don't change too
much" for not doing this in https://github.com/emilk/egui/pull/5411, so
I should've just done this from the start.
This was a significant part of the profile for text layout (as it cloned
almost every `Row`, even though it only needed to change a single
boolean).

Before:
<img width="757" height="250" alt="image"
src="https://github.com/user-attachments/assets/d1c2afd1-f1ec-4cf5-9d05-f5a5a78052df"
/>

After:
<img width="615" height="249" alt="image"
src="https://github.com/user-attachments/assets/c70966da-c892-4e84-adba-494d0f37f263"
/>

(note that these profiles focus solely on the top-level
`Galley::layout_inline` subtree, also don't compare sample count as the
duration of these tests was completely arbitrary)

egui_demo_lib `*text_layout*` benches:
<img width="791" height="461" alt="image"
src="https://github.com/user-attachments/assets/4f97ce84-2768-4876-9488-d42f8f358ed1"
/>

* [X] I have followed the instructions in the PR template

(As usual, the tests fail for me even on master but the failures on
master and with these changes seem the same :))

crates/egui/src/text_selection/visuals.rs

@@ -31,7 +31,8 @@ pub fn paint_text_selection(
     let max = galley.layout_from_cursor(max);
 
     for ri in min.row..=max.row {
-        let row = Arc::make_mut(&mut galley.rows[ri].row);
+        let placed_row = &mut galley.rows[ri];
+        let row = Arc::make_mut(&mut placed_row.row);
 
         let left = if ri == min.row {
             row.x_offset(min.column)
@@ -41,7 +42,7 @@ pub fn paint_text_selection(
         let right = if ri == max.row {
             row.x_offset(max.column)
         } else {
-            let newline_size = if row.ends_with_newline {
+            let newline_size = if placed_row.ends_with_newline {
                 row.height() / 2.0 // visualize that we select the newline
             } else {
                 0.0