documents: format always reads apart; read cutoff back to 128 KiB

The format command never parses on the UI thread; refusals past the
size checks come by event. The document's at-once read returns to
128 KiB, so a visual opening between the cutoffs starts at the top.
Timing test takes web_time::Instant.

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

src/documents/document/mod.rs

@@ -41,7 +41,7 @@ use crate::{
 };
 
 /// Texts this long parse at once; longer ones elsewhere.
-const AT_ONCE: usize = 16 << 10;
+const AT_ONCE: usize = 128 << 10;
 
 /// Longer texts are never read: source alone.
 pub const LARGEST_READ: usize = 2 << 20;