fix: guard against None stdout in fix_console_encoding for pythonw.exe

When launched via pythonw.exe (no console), sys.stdout/stderr are None.
Accessing .encoding on None raised AttributeError, caught by the GUI's
pipeline import guard and shown as 'Failed to load pipeline'. Added
None check before the encoding check in fix_console_encoding(), added
a test, and documented the pitfall in CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

arma_modlist_tools/compat.py

@@ -102,6 +102,8 @@ def fix_console_encoding() -> None:
     """
     if not is_windows():
         return
+    if sys.stdout is None or sys.stderr is None:
+        return
     if sys.stdout.encoding and sys.stdout.encoding.lower() == "utf-8":
         return
     sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding="utf-8", errors="replace")