feat(streams): trace first WebSocket writes (#4074)

## Summary & Motivation

Tags the first write of a session, and the first write on each reconnect, with phase timings on the existing `workflow.stream.write` span — token/config resolution, connect, wait-for-open, send, and ack round trip — so cold WebSocket write latency can be attributed to a client-side phase before changing first-chunk transport behavior. Later writes keep the two attributes they had, since the per-phase clocks only mean anything while a connection is being established.

## Test Plan

Unit tests added; existing world-vercel suite and typecheck pass.