Publish formal evidence schema specification

[PawanRamaMali]

Summary

The evidence schema (columns, types, valid values, pillar assignments) is currently implicit in the code. It should be formally documented and machine-readable.

Tasks

• Create a JSON Schema definition for the evidence data frame
• Document all valid pillar names, indicator codes, and value ranges
• Add vignette("evidence-schema") with the full specification
• Export a validate_evidence() function that checks a data frame against the schema and returns informative errors

Why

External tools and pharma partners need a clear contract for what evidence format R4SUB expects. This is a prerequisite for parser development and SCI validation work.