feat(streams): add writer session seam (#3832)

## Summary & Motivation

Gives one in-memory stream writer a stable identity and its own sequence space, so a transport can preserve chunk ordering across a mid-stream HTTP/WebSocket transition. `Streamer.streams.createWriteSession` is optional — Worlds that don't implement it keep using `write`/`writeMulti`/`close` unchanged.

Abort disposes the session rather than closing it, since a producer failure is transport cleanup, not stream completion.

## Test Plan

Tests added, plus the full `@workflow/world-vercel` suite and package builds/typecheck pass. Root build/typecheck is blocked locally by a missing Rust toolchain for the unrelated `@workflow/swc-plugin`.