Code editor: columns sit where gpui lays them out

Carets, selections, presses and the input method box measured the line
as columns times one 'm', so a wide character (CJK, emoji) put the caret
mid-glyph. editor::measure lays out the drawn line; rulers and indent
guides stay on the column grid; whitespace marks hide the rest.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/editor/mod.rs

@@ -16,6 +16,7 @@ mod keybindings;
 mod keys;
 mod layout;
 mod line;
+mod measure;
 mod moves;
 mod preview;
 mod problems;