fix: WITH clause scoping and filtering edge cases

[DecisionNerd]

Summary

Phase 3 of v0.3.8 TCK compliance push. WITH clause has ~104 failing TCK scenarios.

Failure categories (from TCK analysis)

• WITH WHERE filtering: predicate applied after projection (variable scope)
• WITH DISTINCT: deduplication of projected values
• Chained WITH: multiple WITH clauses, variable scoping between them
• WITH after aggregation: scope after aggregate projection
• Variable scoping: variables before WITH not visible after unless projected

TCK Impact

~104 scenarios (largest remaining failure category)

Acceptance Criteria

• WITH n WHERE n.x > 0 filters correctly after projection
• WITH DISTINCT x deduplicates projected values
• Variables from before WITH are not visible after unless projected
• Chained WITH clauses maintain correct scope
• Subscript expressions on WITH-projected variables work correctly