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>

lib/flow/scoreboard.json

@@ -1,6 +1,6 @@
 {
-  "total": 162,
-  "passed": 162,
+  "total": 166,
+  "passed": 166,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
@@ -13,7 +13,7 @@
     "railway": { "passed": 10, "total": 10 },
     "integration": { "passed": 10, "total": 10 },
     "hygiene": { "passed": 9, "total": 9 },
-    "host": { "passed": 11, "total": 11 }
+    "host": { "passed": 15, "total": 15 }
   },
   "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "done", "phase5": "done", "phase6": "done", "phase7": "done", "phase8": "done" }
 }