textframe(style): Use namespace import for difflib

why: Follow CLAUDE.md guideline for stdlib namespace imports
what:
- Change from difflib import ndiff to import difflib
- Use difflib.ndiff() instead of ndiff()

Author: tony

src/libtmux/textframe/plugin.py

@@ -11,8 +11,8 @@
 
 from __future__ import annotations
 
+import difflib
 import typing as t
-from difflib import ndiff
 
 import pytest
 from syrupy.assertion import SnapshotAssertion
@@ -120,7 +120,7 @@ def pytest_assertrepr_compare(
     if left_render != right_render:
         lines.append("")
         lines.append("Content diff:")
-        lines.extend(ndiff(right_render, left_render))
+        lines.extend(difflib.ndiff(right_render, left_render))
 
     return lines