Refactor runtime-core artifact verifier

[chubes4]

Summary

• Move artifact bundle verification implementation out of packages/runtime-core/src/index.ts into artifact-bundle-verifier.ts.
• Keep public exports stable by re-exporting the verifier module from index.ts.
• Cut the runtime-core entrypoint roughly in half so it is closer to a public API surface.

Closes #375

Verification

• npm run build
• npm run artifact-bundle-verifier-smoke
• npm run artifact-contract-smoke
• npm run package-distribution-smoke
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Inspecting the remaining runtime-core entrypoint surface, extracting the artifact verifier module, and running verification. Chris remains responsible for review and merge.