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_engine.py

@@ -7,25 +7,26 @@ from .fold_types import FOLD_NODES
 
 
 
+def visit_node(fold_types, node, ranges):
+    if node.type in fold_types:
+        start_line = node.start_point[0]
+        end_line   = node.end_point[0]
+
+        if end_line > start_line:
+            ranges.append({
+                "start_line": start_line,
+                "end_line": end_line,
+                "id": (start_line, end_line, node.type),
+            })
+
+    for child in node.children:
+        visit_node(fold_types, child, ranges)
+
 def get_folding_ranges(lang_name: str, ast):
     root       = ast.root_node
     fold_types = FOLD_NODES.get(lang_name, set())
     ranges     = []
 
-    def visit(node):
-        if node.type in fold_types:
-            start_line = node.start_point[0]
-            end_line   = node.end_point[0]
-
-            if end_line > start_line:
-                ranges.append({
-                    "start_line": start_line,
-                    "end_line": end_line,
-                    "id": (start_line, end_line, node.type),
-                })
-
-        for child in node.children:
-            visit(child)
-
-    visit(root)
+    visit_node(fold_types, root, ranges)
     return ranges
+