Implement native support for lag window function

**Describe**
Spark `lag(...)` 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 `lag(...)` 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