flow: reference host driver flow-drive-host/flow-run-host + 4 tests

Completes the host ABI from work-queue to driver loop: the host supplies only a
(kind payload) -> answer dispatch fn; flow-drive-host services one tick of pending
requests, flow-run-host ticks until quiescent (bounded). Tested via the art-dag
render -> human-review -> publish pipeline driven entirely by flow-run-host. The
art-dag integration is now: define dispatch, call flow-run-host. 166/166, 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` → **162/162** (Phases 1-8 complete; host ABI for art-dag)
+`bash lib/flow/conformance.sh` → **166/166** (Phases 1-8 complete; host ABI + reference driver)
 
 ## Ground rules
 
@@ -175,8 +175,11 @@ points later without reverse-engineering tag shapes. `lib/flow/host.sx`.
 - [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).
+- [x] `(flow-drive-host dispatch)` / `(flow-run-host dispatch maxticks)` — reference
+  host driver: the host supplies only a `(kind payload) -> answer` dispatch fn; the
+  loop drains pending requests and resumes until quiescent (bounded).
+- [x] `lib/flow/tests/host.sx` — 15 cases incl. the art-dag-shaped driver loop
+  (render → human-review → publish) run both manually and via `flow-run-host`.
 - 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