Add artifact reference normalization helpers

[chubes4]

Summary

• Adds runtime-core helpers for canonical artifact reference paths, digest normalization, content type/mime normalization, manifest file refs, trace refs, artifact bundle refs, observation refs, and browser artifact summary refs.
• Reuses those helpers in runtime episode/reference/run registry code paths so consumers do not need to hard-code the same shapes.
• Adds a focused smoke fixture for the new normalization API.

Verification

• npm run build
• npm run artifact-reference-normalization-smoke
• npm run artifact-contract-smoke
• npm run runtime-episode-smoke
• npm run run-registry-smoke

Notes

• Refs Add generic block editor action and evidence primitives #460
• Related to Add caller-owned browser artifact bundle normalization #453, but this PR stays limited to generic artifact/ref normalization helpers rather than caller-owned bundle validation.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the runtime-core helper implementation, smoke coverage, and focused rewiring; Chris remains responsible for review and testing.