fix(core): drain step stream writes before completion (#3941)

## Summary & Motivation

### Situation

- Workflow stream writers are expected to call `releaseLock()` when a step finishes writing so another step can acquire the stream.
- The runtime observes that release and drains the server sink, but `step_completed` currently races the overall stream operation against 500ms.
- A slow PUT can therefore continue under `waitUntil` while the next step starts and reads a stale tail.
- Release can also happen while native `writer.write()` promises remain unsettled, leaving frames upstream of the server sink when a naive drain runs.
- Writers intentionally kept locked must remain non-blocking so producer and consumer steps can overlap.

### Fix

- Treat a writer released before step return as an implicit durable handoff boundary.
- At step end, acquire the unlocked stream with a temporary writer and enqueue an internal checkpoint behind all writes queued by the released writer.
- Once the checkpoint crosses serialization, wait for those frames to reach the server sink and drain the group-commit PUT before `step_completed`.
- If the writer remains locked, do not wait for durability; preserve the existing 500ms inline-loop heuristic and background `waitUntil` lifecycle.
- Drain failures or the 30-second safety timeout fail/retry the step. Client disconnect errors remain non-fatal.

## Test Plan

- Covers released and held locks, release with unsettled writes, delayed first writer acquisition, forwarded writable arguments, drain timeout/failure, and multiple streams.
- `pnpm --filter @workflow/core build`
- `pnpm --filter @workflow/core typecheck`
- `pnpm --filter @workflow/core test` — 2,405 passed, 3 expected failures, 1 skipped