Documents: untouched cells keep their bytes; open cells take padding presses

A trailing backslash doubles only before a closing pipe. A write at a
block's start keeps the block, so Tab leaves a first cell without outer
pipes. Padding of the open cell moves its caret to the end, and a right
press there points at the end.

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

src/documents/document/visual.rs

@@ -175,7 +175,7 @@ impl MarkdownDocument {
         cx.listener(|document, event: &MouseDownEvent, window, cx| {
             window.prevent_default();
             cx.stop_propagation();
-            document.point_open(event.position, cx);
+            document.point_open(Some(event.position), cx);
         })
     }