Expose artifact-backed viewport capture in browser SDK

[chubes4]

Summary

• add generic viewport screenshot capture and verification to window.wpCodeboxBrowser.v1
• send capture through a bounded generic browser invoker for caller-selected route and viewport
• persist PNG bytes through Codebox persist-browser-artifact by default
• return canonical immutable artifact refs and server-produced SHA-256 evidence
• verify existence and checksum through Codebox inspect-artifact
• fail closed for invalid routes, capture errors, timeout, missing PNG/artifact, and checksum mismatch
• advertise capture and verification through SDK capability discovery

Verification

• npm run build
• npm run test:browser-runtime-generic-invoker
• npm exec -- tsx tests/browser-sdk-facade.test.ts
• npm exec -- tsx tests/browser-artifact-session.test.ts
• npm exec -- tsx tests/artifact-result-envelope.test.ts
• git diff --check origin/main...HEAD

Closes #2262

AI assistance

OpenAI GPT-5.6 Sol via OpenCode general coding subagents was used to inspect the browser SDK, browser runtime, artifact ability contracts, implement the feature, and add tests. OpenCode was also used for architecture review and independent verification. Chris Huber reviewed and remains responsible for every line.