Use strongly typed CharIndex and ByteIndex + bug fixes (#8245)

Less risk of confusing the two.

Found and fix a couple real bugs in the process!

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/epaint/src/text/cursor.rs

@@ -1,5 +1,7 @@
 //! Different types of text cursors, i.e. ways to point into a [`super::Galley`].
 
+use super::index::CharIndex;
+
 /// Character cursor.
 ///
 /// The default cursor is zero.
@@ -7,7 +9,7 @@
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct CCursor {
     /// Character offset (NOT byte offset!).
-    pub index: usize,
+    pub index: CharIndex,
 
     /// If this cursors sits right at the border of a wrapped row break (NOT paragraph break)
     /// do we prefer the next row?
@@ -18,9 +20,9 @@ pub struct CCursor {
 
 impl CCursor {
     #[inline]
-    pub fn new(index: usize) -> Self {
+    pub fn new(index: impl Into<CharIndex>) -> Self {
         Self {
-            index,
+            index: index.into(),
             prefer_next_row: false,
         }
     }
@@ -83,5 +85,5 @@ pub struct LayoutCursor {
     /// Character based (NOT bytes).
     /// It is fine if this points to something beyond the end of the current row.
     /// When moving up/down it may again be within the next row.
-    pub column: usize,
+    pub column: CharIndex,
 }