Align browser artifact projection refs

[chubes4]

Summary

• preserve explicit artifactRefs when runtime-core builds browser artifact persistence projections
• align nested content_digest digest handling with the browser-runtime projection helper
• compare runtime-core projection output against the browser SDK facade for the same persistence input

Verification

• npm run test:browser-sdk-facade
• npm run test:browser-callback-materialization-contracts
• npm run test:artifact-reference-dtos
• npm run build
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Auditing browser/runtime artifact projection drift, applying the runtime-core parity fix, and running verification.