Add partial replay runtime snapshot bundles

[chubes4]

Summary

• Add partial-replay and replayable-runtime-state snapshot semantics to runtime episode artifacts.
• Emit opt-in files/runtime-snapshots/<snapshot>.json bundles with replay instructions, artifact refs, SHA-256 hashes, and explicit WordPress-state limitations.
• Verify snapshot artifact refs from the runtime reference manifest and extend runtime episode smoke coverage.

Closes #220.

Tests

• npm run check
• After rebasing onto origin/main: npm run build
• After rebasing onto origin/main: npm run runtime-episode-smoke
• After rebasing onto origin/main: npm run artifact-bundle-verifier-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the snapshot bundle contract, updated smoke coverage, and ran verification; Chris remains responsible for review and merge.