Specify provider icon namespaces

[konojunya]

Summary

• define provider:icon-name alongside existing unnamespaced theme icons
• preserve authored node kinds and existing STK5001 fallback behavior
• require explicit local provider packs and separate provider terms/provenance
• update normalized IR validation and add a valid conformance fixture
• record the public architecture decision in ADR 0005

Verification

• JSON Schema metaschema validation
• normalized IR and diagnostic fixture validation
• compiler diagnostic coverage validation
• formatter fixture validation
• npm run test:language
• npm run typecheck:language
• npm run pack:check