Live preview: select, delete and word-delete across lines

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -192,7 +192,9 @@ file time as its date, and is then encrypted (name it with lower-case letters, n
   built for millions.
 - Pages are only reloaded from disk when Mako starts.
 - Live preview works a line at a time (a code block counts as one), so a table or a quote spread over
-  several lines shows rendered line by line. View shows the whole page as one.
+  several lines shows rendered line by line. View shows the whole page as one. Selecting across lines
+  (Ctrl+A, Shift+Up or Down, Shift+click, or dragging) opens those whole lines as raw markdown, so a
+  selection that starts or ends partway through a line takes in the whole line.
 
 ## Design