fix(lint): ignore UP042 str+Enum rule, fix import sorting

UP042 (use StrEnum) flagged by newer ruff in CI. Ignored for now
as migrating all enums requires incremental changes.

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

pyproject.toml

@@ -70,6 +70,7 @@ ignore = [
     "N818",    # exception name should end with Error — existing convention
     "F821",    # undefined name — false positives on forward-reference type hints
     "F822",    # undefined name in __all__ — lazy-loaded module attributes
+    "UP042",   # str+Enum → StrEnum — requires Python 3.11+ migration, do incrementally
 ]
 
 # Allow autofix for all rules