Fix: Improve code folding functionality and gutter rendering

- Updated folding actions and engine for more consistent behavior.
- Added helper function `is_fold_hidden()` to check fold visibility state.
- Improved gutter renderer to handle collapsible code blocks more reliably.
- Refined tag handling for invisible folds to prevent desync issues.
- Removed code fold Fix related entry in `TODO.md`.

plugins/code/ui/code_fold/folding_actions.py

@@ -9,7 +9,7 @@
 def collapse_range(view, fold):
     buffer = view.get_buffer()
     start  = buffer.get_iter_at_line(fold["start_line"] + 1)
-    end    = buffer.get_iter_at_line(fold["end_line"] + 1)
+    end    = buffer.get_iter_at_line(fold["end_line"]   + 1)
 
     buffer.apply_tag_by_name("invisible", start, end)
 
@@ -17,6 +17,8 @@ def collapse_range(view, fold):
 def expand_range(view, fold):
     buffer = view.get_buffer()
     start  = buffer.get_iter_at_line(fold["start_line"] + 1)
-    end    = buffer.get_iter_at_line(fold["end_line"] + 1)
+    end    = buffer.get_iter_at_line(fold["end_line"]   + 1)
 
     buffer.remove_tag_by_name("invisible", start, end)
+
+