Documents: an open block keeps its look

The open field sits in the closed block's frame at its size, line
height and weight; headings keep their step. Code and math keep their
panel. Typewriter centering also keys on the ends' room, so a block
opened at the same height still centers.

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

src/documents/document/mod.rs

@@ -117,7 +117,7 @@ pub struct MarkdownDocument {
     /// Typewriter room past each end of the list.
     room: Pixels,
     /// The block, text, typing and heights last centered.
-    centered: Option<(usize, u64, u64, [u64; 2])>,
+    centered: Option<(usize, u64, u64, [u64; 3])>,
     focus: FocusHandle,
     /// A secondary press took focus.
     pointing: bool,