ROW_NUMBER() OVER () or SUM(x) OVER () without PARTITION BY computes
over the entire result set. Sometimes intentional, often a bug - the
author forgot to specify the partition key.

Should fail

SELECT
  user_id,
  ROW_NUMBER() OVER () AS rn
FROM events;

Should pass

SELECT
  user_id,
  ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY event_time) AS rn
FROM events;

Implementation hints

• Structural rule in sql_guard/rules/structural.py.
• Model after S002 deeply-nested-subquery - uses sqlparse tokens.
• Detect OVER ( ... ) where the inner content lacks PARTITION BY.
• Severity: warning.

Estimated LOC: ~40 code + ~25 test.