Runtime resolution of dynamic values (@today, @this-month, etc.) based on field type.

The parser emits {:dynamic, :name} tokens. The semantic analyzer calls Dequel.Dynamic.resolve(type, name) where type is the field's schema type.

Returns either:

• A concrete value (for point-in-time dynamics like @today)
• {:range, start, end} (for range dynamics like @this-month, @this-year)