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/api.sx

@@ -1,8 +1,9 @@
 ;; lib/flow/api.sx — flow runtime entry points.
 ;;
 ;; Builds a Scheme env preloaded with the flow combinators (lib/flow/spec.sx),
-;; the durable store + lifecycle (lib/flow/store.sx), and the fed-sx remote layer
-;; (lib/flow/remote.sx), and provides SX helpers to run flow programs.
+;; the durable store + lifecycle (lib/flow/store.sx), the fed-sx remote layer
+;; (lib/flow/remote.sx), and the host integration ABI (lib/flow/host.sx), and
+;; provides SX helpers to run flow programs.
 ;;
 ;; Scheme-level API (available inside flow programs):
 ;;   (flow/start flow input) — run a flow; raw result if it completes, else
@@ -10,10 +11,11 @@
 ;;   (flow/resume id value)  — resume a suspended flow (store.sx)
 ;;   (flow/cancel id)        — cancel a flow (store.sx)
 ;;   (suspend tag)           — suspension point (spec.sx)
+;;   (request kind payload)  — host request envelope over suspend (host.sx)
 ;;   (remote-node addr fn)   — node executed on a federation peer (remote.sx)
 ;;
 ;; SX-level helpers (for hosts and tests):
-;;   (flow-make-env)        — fresh standard env + combinators + store + remote
+;;   (flow-make-env)        — fresh standard env + combinators + store + remote + host
 ;;   (flow-run src)         — eval a Scheme program string in a reset shared env
 ;;   (flow-run-in env src)  — eval a Scheme program string in a given env
 ;;
@@ -31,6 +33,7 @@
       (flow-load-combinators! env)
       (flow-load-store! env)
       (flow-load-remote! env)
+      (flow-load-host! env)
       env)))
 
 (define