Publish the curated Stack example catalog

[konojunya]

Summary

• define a versioned JSON Schema and catalog for reusable Stack examples
• expand the corpus to nine sources across starter, intermediate, and advanced stages, including AWS, GCP, Azure, SaaS, and mixed-provider architectures
• record purpose, providers, represented syntax features, expected structure, and accessible thumbnail descriptions for every example
• validate catalog/source coverage, structural counts, feature metadata, provider metadata, and required coverage in CI
• replace stale built-in icon IDs in the existing examples with current canonical IDs

Verification

• JSON Schema metaschema and catalog validation
• python scripts/validate-example-catalog.py
• normalized IR and diagnostic schema validation
• compiler diagnostic and formatter fixture validation
• npm run test:language
• npm run typecheck:language
• npm run pack:check
• check and render all nine examples with Stack CLI 0.3.0