feat(streams): add WebSocket writer lifecycle (#3833)

## Summary & Motivation

Implements the client half of `workflow-stream-ws/v1` behind the existing default-off `WORKFLOW_STREAMS_TRANSPORT=ws` gate, populating the `createWriteSession` seam only when opted in.

- Writes and closes are serialized over one socket per writer lifetime; groups above the v1 per-request chunk cap are split without resetting writer-local sequence.
- Any failure before the upgrade is accepted (declined upgrade, proxy, load error, a dispatch that beats the handshake) falls back to HTTP for the rest of the writer's life.
- Once a write is on the socket, a missing or uncorrelatable reply poisons the session rather than replaying over HTTP, since a duplicate append cannot be ruled out.
- Idle clean closes reconnect with the same writer id, capped at three attempts so a draining server can't hot-loop.
- The handshake gets a `workflow.stream.ws.connect` span and each frame a synthesized `http POST` span, so per-event tracing survives the non-`fetch` transport.

## Test Plan

New unit tests cover the lifecycle, fallback, and poisoning paths; 598 `@workflow/world-vercel` tests plus package build/typecheck and workspace lint/format pass. Root build/typecheck couldn't run locally (missing Rust toolchain for the unrelated `@workflow/swc-plugin`).