docs(engineering): mark Epic 3 closed (3-6 shipped in v0.7)

§8 now records the extension-slot docs as shipped in v0.7 and notes
Epic 3 closed. The remaining roadmap collapses to Epic 6 (ship v1.0)
plus Epic 5's already-shipped observability work.

Author: lesnik512

planning/engineering.md

@@ -131,7 +131,8 @@ Post-pivot, the roadmap has three categories. Topic slugs in `planning/specs/` a
 - **Epic 3 — Resilience:**
   - **Shipped in v0.4 slice 1:** `Retry` middleware + Finagle-style `RetryBudget` token bucket + `attempt_timeout=` parameter (folded-in 3-1). See [`planning/specs/2026-06-05-retry-and-retry-budget-design.md`](specs/2026-06-05-retry-and-retry-budget-design.md) and [`planning/plans/2026-06-05-retry-and-retry-budget-plan.md`](plans/2026-06-05-retry-and-retry-budget-plan.md).
   - **Shipped in v0.4 slice 2:** `Bulkhead` middleware (concurrency limiter via `asyncio.Semaphore` with bounded acquire wait). See [`planning/specs/2026-06-05-bulkhead-design.md`](specs/2026-06-05-bulkhead-design.md) and [`planning/plans/2026-06-05-bulkhead-plan.md`](plans/2026-06-05-bulkhead-plan.md).
-  - **Remaining:** `3-6` extension-slot docs.
+  - **Shipped in v0.7:** `3-6` extension-slot docs — [`docs/middleware.md`](../docs/middleware.md). Covers the Middleware Protocol, phase decorators, a Request-ID worked example, and "when NOT to write a middleware." See [`planning/specs/2026-06-05-extension-slot-docs-design.md`](specs/2026-06-05-extension-slot-docs-design.md) and [`planning/plans/2026-06-05-extension-slot-docs-plan.md`](plans/2026-06-05-extension-slot-docs-plan.md).
+  - **Epic 3 closed.**
 - **Epic 4 — Streaming:** SHIPPED in v0.5 (PR #…): `AsyncClient.stream()` context manager + Retry refuses streamed-body requests. See [`planning/specs/2026-06-05-streaming-design.md`](specs/2026-06-05-streaming-design.md) and [`planning/plans/2026-06-05-streaming-plan.md`](plans/2026-06-05-streaming-plan.md).
 - **Epic 5 — Observability:** SHIPPED in v0.6 (PR #…) — re-scoped from the original 4-story plan. `Retry` and `Bulkhead` emit operational events via stdlib `logging` + opt-in OpenTelemetry span events. Stories `5-1` (Layer 1 middleware hooks) and `5-4` (standalone OTel middleware) RETIRED — `opentelemetry-instrumentation-httpx` already covers transport-level tracing; a separate httpware middleware would duplicate it. See [`planning/specs/2026-06-05-observability-design.md`](specs/2026-06-05-observability-design.md) and [`planning/plans/2026-06-05-observability-plan.md`](plans/2026-06-05-observability-plan.md).
 - **Epic 6 — Ship v1.0:** `6-2` docs site (`mkdocs`), `6-3` benchmarks, `6-5` release flow (Trusted Publishers + Sigstore).