Phase 8b: Async-Runtime Bridge (`@dexpace/rx`)

[Wahbeh-Mohammad]

Ship @dexpace/rx, the one async-runtime adapter worth building for Node: an RxJS Observable view over Phase
6's Page and SseStream.

Scope

• Backpressure-honoring Observable view of SseStream with the fatal/non-fatal error split and documented
  source ownership (SSE-41, MAY; restated as ASYNC-21).
• Observable view of Page / the item stream.
• Depends only on Phase 6's Page/SseStream — never on Transport.
• Disposition table: a large share of ASYNC-* IDs collapse onto their TRANSPORT-* twin, and
  ASYNC-3/-4/-7/-14 are inapplicable outright — Node has no blocking-transport or worker-thread-pool
  model for them to bite on (the same premise that closed SEAM-18 as "never"). ASYNC-18 is inapplicable to
  the whole port, not merely out of 8b's scope.

Spec refs

Product spec §18 (ASYNC-*). sdk-design: §3.2.

Dependencies

Phases 0–6 (needs Page and SseStream). Independent of 8a — order is convenience only.

Docs

• Design: docs/superpowers/specs/2026-07-28-phase8b-async-runtime-design.md
• Plan: docs/superpowers/plans/2026-07-28-phase8b-async-runtime.md
• Segmentation: docs/superpowers/specs/2026-07-28-phase8-segmentation-design.md
• Validation prompt: docs/validation-prompts/phase8b-async-runtime-validation-prompt.md