Implement native support for lead window function

[weimingdiit]

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

The implementation should preserve Spark semantics for:

• partitioning
• ordering
• offset handling
• default value handling
• null propagation
• interaction with supported window frames