fix: support large files in comparisons

Replace the 16 MiB comparison rejection with independent bounded per-side loading and frozen Working blobs. Bound diff computation, defer large inline highlighting, index source-row navigation, and expose actual load failures. Verified the real 52.3 MB parser through repository/session APIs and terminal rendering with EOF/start navigation.

tests/unit/editor.rs

@@ -135,10 +135,21 @@ fn resolver_shares_native_excerpt_search_motions_and_explicit_writable_side() {
         type_text(editor, "/needle");
         editor.handle(Input::Enter);
         assert_eq!(editor.cursor().row, 70);
-        let rows = editor.diff().unwrap().source_rows(Focus::Before).count();
+        let rows = editor
+            .diff()
+            .unwrap()
+            .source_rows_in(Focus::Before, 0, usize::MAX)
+            .count();
         let cursor = editor.cursor();
         editor.handle(Input::Char(']'));
-        assert!(editor.diff().unwrap().source_rows(Focus::Before).count() > rows);
+        assert!(
+            editor
+                .diff()
+                .unwrap()
+                .source_rows_in(Focus::Before, 0, usize::MAX)
+                .count()
+                > rows
+        );
         assert_eq!(editor.cursor(), cursor);
         type_text(editor, "21g");
         assert_eq!(editor.cursor().row, 20);
@@ -4644,7 +4655,7 @@ fn editable_diff_brackets_expand_caret_excerpt_without_moving_source_anchor() {
             editor
                 .diff()
                 .unwrap()
-                .source_rows(Focus::After)
+                .source_rows_in(Focus::After, 0, usize::MAX)
                 .collect::<Vec<_>>()
         };
         assert_eq!(