Add Param/ConstParam/Returns API for scalar functions

Introduces a new annotation-based API for defining scalar function parameters:
- Param(type, doc) for columnar inputs (arrays from batch)
- ConstParam(type, doc) for constant-folded scalars
- Returns(type) for declaring output type

Also adds runtime type validation for the new API:
- TypeMismatchError raised when input/output types don't match declarations
- AnyArrow parameters skip validation
- Legacy Arg descriptor API continues to work without type checking

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rustyconover