Document supported attribute filter operators

Summary

The attribute filters guide (user_guide/03-attribute-filters.qmd) demonstrates filter usage through examples but doesn't include a reference section listing all supported operators.

A user reading the guide can infer =, >=, AND, and IS NULL from the examples, but there's no way to discover the full set of available operators without reading the source code.

Suggested addition

A "Filter Operators" reference section listing:

Comparison operators (SQL-like string / dict AST):

• = / eq, != / ne
• > / gt, >= / gte, < / lt, <= / lte
• IN / in, NOT IN / nin
• IS NULL, IS NOT NULL

Logical operators:

• AND, OR

Dict AST structure:

• {"type": "<operator>", "key": "<column>", "value": <value>}
• {"type": "and"|"or", "filters": [...]}

This could go between the current "Recap: Declaring Attributes" and "Built-In Backend Columns" sections, or as a new standalone section.

Edit user_guide/03-attribute-filters.qmd between “Recap: Declaring Attributes” and “Built-In Backend Columns.” Review the existing examples, then add a “Filter Operators” reference covering comparison and logical operators plus the specified dict AST structures. Done means readers can discover the supported operators without consulting source code.