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Alex Langenfeld 31dabce0c8 [core] Prefetch stream read encryption keys (#3871 )

## Summary & Motivation

Stream reads now start the run-metadata and encryption-key lookup concurrently with the stream GET, instead of waiting for the first encrypted frame to arrive before touching either, so a reader no longer stalls on that round trip. The key resolver is memoized per readable session and its promise is observed eagerly, so a lookup that fails while nothing consumes it can't surface as an unhandled rejection; the original error still reaches the consumer through the deserialize transform. `getReadable()` builds its underlying stream lazily so an unread stream does no work. The key-resolution phase gets its own `workflow.stream.read.resolve_key` span, keeping `workflow.stream.read`'s time-to-first-chunk measuring the raw stream alone.

## Test Plan

Unit tests added for the concurrent start, the single resolver across reconnects and sessions, and the cancellation/failure paths; `pnpm --filter @workflow/core test` and `typecheck` pass locally.


## Durabench evidence

A completed matched Workflow stream sweep on the final-refactor implementation showed the intended first-chunk improvement. The cell used the canonical Eve cadence, speed 1, in-step reader, five executions per variant, and concurrency 1.

| Metric | Key prefetch | Control |
| --- | ---: | ---: |
| CTT-first p75 | **107 ms** | 167 ms |
| CTT-first p99 | **137 ms** | 189 ms |
| All-chunk CTT p75 | 160 ms | 159 ms |
| All-chunk CTT p99 | 1509 ms | 2260 ms |
| E2E | 54.024 s | 54.046 s |

The feature and control cells both completed successfully. The first-frame improvement does not come with an all-chunk CTT or end-to-end regression in this sample.

Source sweep: `psweep-1788293149845-37d61d53-477c-476c-bd19-a0b7cfa7a2ca`.

- Feature run: `prod-1788378070863-f2435fc6-3aa2-4fd3-9608-71c67d273c9a` at `36a17405`
- Control run: `prod-1788378070866-ecc76dbd-8cfd-4b23-9fd6-386c0fae9007` at `3c087789`

The current head (`d63263ee3`) adds only the reviewed byte-branch completion-state consistency guard; it does not alter the object-stream key-prefetch path measured above.

2026-09-02 16:09:05 -05:00