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/scoreboard.md

@@ -1,6 +1,6 @@
 # flow-on-sx Scoreboard
 
-**All tests pass: 151 / 151 across 10 suites. Phases 1-7 complete.**
+**All tests pass: 162 / 162 across 11 suites. Phases 1-8 complete.**
 
 `bash lib/flow/conformance.sh`
 
@@ -18,6 +18,7 @@
 | railway | 10 | Phase 6: `attempt` — fail-value short-circuiting sequence + recover rejoin |
 | integration | 10 | Phase 7: end-to-end order + onboarding flows composing every phase (suspend, branch, federation, crash recovery, handoff, introspection) |
 | hygiene | 9 | Phase 5: `flow/gc` (prune terminal flows), `flow/forget` (drop one terminal record) |
+| host | 11 | Phase 8: host ABI — `request`/`await-human`/`await-render`, `flow-host-requests` work queue; art-dag-shaped driver loop |
 
 ## Architecture