fix: keep large-file editing responsive

Bound synchronous full-source syntax parsing, cache dirty comparisons, edit ordinary inline changes in place, retain changed-row undo history, and buffer atomic saves. Skip repository snapshots beyond existing worker limits before copying their text.

src/gutter.rs

@@ -19,7 +19,7 @@ use std::{
     thread::{self, JoinHandle},
 };
 
-const FILE_LIMIT: usize = 16 * 1024 * 1024;
+pub(crate) const FILE_LIMIT: usize = 16 * 1024 * 1024;
 const BATCH_LIMIT: usize = 64 * 1024 * 1024;
 const FILE_COUNT_LIMIT: usize = 1024;
 type Files = Vec<(usize, PathBuf, Arc<str>)>;