feat(live): add WebSocket transport seam

[benvinegar]

Summary

• add an onEvent feed tap to createApp and export FeedEvent for hosts
• add liveTransport: "ws" to the embeddable viewer host contract while keeping SSE as the default
• implement WS reconnect/resync, heartbeat, and cleanup, with unit/e2e coverage

Notes

• The OSS server still does not implement a WebSocket upgrade route; hosts own that transport and can use Cloudflare Durable Object WebSocket hibernation.
• Optional follow-up: add a default Worker WebSocket handler for self-hosted Workers deployments.

Tests

• npm run typecheck
• npm test -- --test-name-pattern 'onEvent'\n- npx playwright test e2e/embed-ws.spec.ts --project=chromium\n- npm run format:check\n- npm run lint\n\nThis PR description was generated by Pi using OpenAI GPT-5