documents: block commands reread their result

Fixes R4.7 round 2: a heading change or split is parsed again and
refused as Misread when its text, formats or kind would change;
closing hashes and block openers are escaped at split ends; task
boxes below a marker stay; indented code is written from its value;
alerts unwrap past blank and lazy lines; delete counts every gone
opener.

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

src/documents/document/format.rs

@@ -48,6 +48,8 @@ pub enum EditRefusal {
     Busy,
     /// The text changed before the edit was read.
     Changed,
+    /// The result would read as other text or blocks.
+    Misread,
 }
 
 /// The edits and the selection after them.