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>

lib/flow/conformance.sh

@@ -31,6 +31,7 @@ SUITES=(
   "railway     flow-rail-tests-run!      lib/flow/tests/railway.sx"
   "integration flow-int-tests-run!       lib/flow/tests/integration.sx"
   "hygiene     flow-hyg-tests-run!       lib/flow/tests/hygiene.sx"
+  "host        flow-hst-tests-run!       lib/flow/tests/host.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT
@@ -49,6 +50,7 @@ emit_eval () { echo "(epoch $EPOCH)"; echo "(eval \"$1\")"; EPOCH=$((EPOCH+1));
   emit_load "lib/flow/spec.sx"
   emit_load "lib/flow/store.sx"
   emit_load "lib/flow/remote.sx"
+  emit_load "lib/flow/host.sx"
   emit_load "lib/flow/api.sx"
   for SUITE in "${SUITES[@]}"; do
     read -r _NAME _RUNNER FILE <<< "$SUITE"