Implement native support for cume_dist window function

[weimingdiit]

Describe
Spark cume_dist() is not supported in Auron's native window execution path, so queries using it fall back instead of being executed natively.

Describe the solution you'd like
Add native support for Spark cume_dist() in Auron's window execution.

The implementation should preserve Spark semantics for:

• partitioning
• ordering
• peer row handling
• return type and boundary behavior