Refactor runtime reference helpers

[chubes4]

Summary

• Extract runtime reference manifest and replay index constants, types, builders, and digest helpers from packages/runtime-core/src/index.ts into runtime-reference.ts.
• Re-export the new module from the runtime core entrypoint so existing public imports continue to work.
• Keep runtime episode runner behavior in index.ts, with artifact verification still using the same reference digest helpers.

Tests

• npm run build
• npm run runtime-episode-smoke
• npm run runtime-snapshot-restore-smoke
• npm run runtime-action-adapter-smoke
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the focused runtime-core extraction, ran verification, and prepared this PR for Chris to review.