Add Traverse HTTP runtime client and execution state machine

[enricopiovesan]

Summary

• Adds `src/client/traverseClient.ts` — thin HTTP boundary: `execute`, `pollExecution`, `fetchTrace` per spec `033-http-json-api`
• Adds `src/hooks/useExecution.ts` — state machine: idle → loading → polling → succeeded / failed
• Wires `App.tsx` to the client; replaces stub `handleStartWorkflow` and fixes health endpoint to `/healthz` per spec 033
• 11 unit tests across client, hook, and App

Closes #3

Definition of Done

• React can start a workflow through a public Traverse runtime/client boundary
• React subscribes to correlated runtime events (polls executions + fetches trace)
• UI state is driven by events (idle → loading → polling → succeeded/failed)
• Failure, loading, progress, and final states are visible in the UI
• No private Traverse internals imported

Validation

npm run typecheck  # pass
npm run lint       # pass
npm run test       # 11 tests pass (3 files)