Implement native support for nth_value window function

[weimingdiit]

Describe
Spark nth_value(...) 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 nth_value(...) in Auron's window execution.

The implementation should preserve Spark semantics for:

• partitioning
• ordering
• nth index handling
• null handling
• supported frame behavior