flow: host integration ABI (request/await/host-queue) + 11 tests (Phase 8)

The seam for hooking flow to art-dag and human-in-the-loop later. (request kind
payload) suspends with a typed (flow-request kind payload) envelope and returns the
host's resume value; await-human/await-render sugar. (flow-host-requests) is the
host work queue: (id kind payload) for every suspended flow awaiting a host effect;
request?/request-kind/request-payload parse a tag. Tests include the art-dag-shaped
driver loop (render -> human-review -> publish). Host owns IO+persistence; flow only
requests (replay-safe). 162/162 across 11 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **151/151** (Phases 1-7 complete; +store hygiene)
+`bash lib/flow/conformance.sh` → **162/162** (Phases 1-8 complete; host ABI for art-dag)
 
 ## Ground rules
 
@@ -162,6 +162,27 @@ Make the `(fail reason)` value channel compose into real validation/ETL pipeline
 - [x] `lib/flow/tests/railway.sx` — 10 cases: fail short-circuiting, no-run-after-
   failure, recover rejoin, validation pipeline reporting the failing stage
 
+## Phase 8 — Host integration ABI (art-dag / human-in-the-loop)
+
+`suspend` is the seam to the outside world, but a bare tag is an ad-hoc convention.
+This phase defines a stable request/response contract a host (an art-dag driver, a
+review UI) codes against — so flow can orchestrate art-dag with human decision
+points later without reverse-engineering tag shapes. `lib/flow/host.sx`.
+
+- [x] `(request kind payload)` — suspend with a typed `(flow-request kind payload)`
+  envelope; evaluates to the host's resume value. `await-human`/`await-render`/
+  `await-effect` sugar.
+- [x] `(flow-host-requests)` — the host work queue: `(id kind payload)` for every
+  suspended flow waiting on a host request; `request?`/`request-kind`/
+  `request-payload` parse a tag.
+- [x] `lib/flow/tests/host.sx` — 11 cases incl. the art-dag-shaped driver loop
+  (render → human-review → publish, driven by polling the queue + resume).
+- Contract (documented in `host.sx` + README): the host owns IO + persistence; a
+  flow never does IO, it only `request`s; the host performs the effect and feeds the
+  result back via resume (logged, so not re-run on recovery). NOT done here (host
+  side, out of `lib/flow` scope): the real Celery/IPFS bridge and a persistent store
+  backend — those live in the art-dag integration, coding against this ABI.
+
 ## Phase 7 — End-to-end integration
 
 Prove the phases compose: realistic flows exercising attempt + suspend + branch +