Add youaskm3 Traverse HTTP runtime client

[enricopiovesan]

Summary

Implement the thin HTTP client boundary in apps/youaskm3/web-react that lets the React UI start Traverse capability executions and poll for results, using only public spec-033 surfaces.

Why

youaskm3 needs to communicate with Traverse through the approved HTTP+JSON API. The client must be a thin boundary — no private Traverse internals, no business logic, no fake responses.

Definition of Done

• Client reads .traverse/server.json for base_url and workspace_default
• POST /v1/workspaces/{workspace_id}/execute implemented
• GET /v1/workspaces/{workspace_id}/executions/{execution_id} polling implemented
• GET /v1/workspaces/{workspace_id}/traces/{execution_id} fetch implemented
• UI state: loading → polling → succeeded / failed
• No private Traverse internals imported
• Unit tests cover client state machine

Governing spec

033-http-json-api (approved v1.1.0)

Depends on

#11 (youaskm3 UI shell), #3 (traverse-starter client — reuse patterns)