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/mod.rs

@@ -246,7 +246,7 @@ impl MarkdownDocument {
         let written = match open.append {
             true if typed.is_empty() => String::new(),
             true => format!("{}{typed}", joint(text)),
-            false if open.cell.is_some() => cells::kept(typed),
+            false if open.cell.is_some() => cells::kept(typed, cells::closed(text, open.range.end)),
             false => typed.to_string(),
         };
         [&text[..open.range.start], &written, &text[open.range.end..]].concat()