Expose country variable metadata through the public API

[anth-volk]

Fixes #504

Summary

• expose selected country-model variable metadata through the public Variable model
• normalize references and country metadata into JSON-safe values
• populate the fields through the shared country-model loader while preserving compatibility with older models
• exclude empty organizational parameter nodes from the public list and name lookup
• cover synthetic, UK, and US variables and parameter nodes

Tests

• uv run pytest -q (811 passed)
• uv run --no-sync make lint