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.

tests/unit/session.rs

@@ -4,6 +4,47 @@ use std::{
     sync::atomic::{AtomicU64, Ordering},
 };
 
+#[test]
+fn oversized_repository_text_skips_snapshots_and_recovers_after_shrinking() {
+    let fixture = Fixture::new();
+    let path = fixture.file("buffer.txt", "small\n");
+    let mut session = EditorSession::open(Some(path.clone()), vec![], None).unwrap();
+    let buffer = session.editor.buffer_id();
+    session.editor.set_gutter_changes(
+        buffer,
+        session.editor.text_revision(),
+        &path,
+        vec![Some(crate::gutter::Change::Added)],
+    );
+    assert_eq!(
+        session.editor.gutter_change(0),
+        Some(crate::gutter::Change::Added)
+    );
+    fs::write(&path, "x".repeat(crate::gutter::FILE_LIMIT + 1)).unwrap();
+    assert!(session.editor.refresh_files());
+    session.poll_gutter_at(Instant::now());
+    session.synchronize_documents();
+    let source = session
+        .gutter_sources
+        .iter()
+        .find(|source| source.buffer == buffer)
+        .unwrap();
+    assert!(source.contents.is_none());
+    assert!(session.blame_worker.is_none());
+    assert_eq!(session.editor.gutter_change(0), None);
+    fs::write(&path, "small again\n").unwrap();
+    assert!(session.editor.refresh_files());
+    session.poll_gutter_at(Instant::now());
+    session.synchronize_documents();
+    let source = session
+        .gutter_sources
+        .iter()
+        .find(|source| source.buffer == buffer)
+        .unwrap();
+    assert_eq!(source.contents.as_deref(), Some("small again\n"));
+    assert!(session.blame_worker.is_some());
+}
+
 #[test]
 fn resolver_load_errors_return_through_real_session_without_touching_the_draft() {
     let fixture = Fixture::new();