host: Phase 1 — router + handler + GET /feed endpoint on Dream, 28/28

First migrated endpoint onto the SX host. lib/host is a thin wiring layer:
a host handler is a Dream handler (request->response) that calls a subsystem
public API and serialises via a shared JSON envelope.

- handler.sx: host/ok, host/ok-status, host/error, host/json-status (Dream's
  dream-json is 200-only), host/query-int
- router.sx: host/make-app assembles per-domain route groups + /health probe
  into one dream-router (reuses dr/flatten-routes)
- feed.sx: GET /feed reads feed/all + stream combinators, recent-first, with
  ?actor= filter and ?limit= cap
- 3 test suites incl. a golden test (body == subsystem recent stream + envelope)
- conformance.sh mirrors lib/dream's runner

Builds on dream-on-sx (merged, gate green 480/480) rather than a throwaway
native request model; collapses most of plan Phase 4 into Phase 1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/host/tests/handler.sx

@@ -0,0 +1,86 @@
+;; lib/host/tests/handler.sx — host JSON envelope + request-reading helpers.
+
+(define host-hd-pass 0)
+(define host-hd-fail 0)
+(define host-hd-fails (list))
+
+(define
+  host-hd-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! host-hd-pass (+ host-hd-pass 1))
+      (begin
+        (set! host-hd-fail (+ host-hd-fail 1))
+        (append! host-hd-fails {:name name :actual actual :expected expected})))))
+
+;; ── host/ok ────────────────────────────────────────────────────────
+(host-hd-test "ok status 200" (dream-status (host/ok "x")) 200)
+(host-hd-test
+  "ok content-type json"
+  (dream-resp-header (host/ok "x") "content-type")
+  "application/json")
+(host-hd-test
+  "ok envelope ok:true"
+  (contains? (dream-resp-body (host/ok "x")) "\"ok\":true")
+  true)
+(host-hd-test
+  "ok envelope carries data"
+  (contains? (dream-resp-body (host/ok "hi")) "\"data\":\"hi\"")
+  true)
+
+;; ── host/ok-status ─────────────────────────────────────────────────
+(host-hd-test "ok-status custom" (dream-status (host/ok-status 201 "y")) 201)
+(host-hd-test
+  "ok-status data"
+  (contains? (dream-resp-body (host/ok-status 201 "y")) "\"data\":\"y\"")
+  true)
+
+;; ── host/error ─────────────────────────────────────────────────────
+(host-hd-test "error status" (dream-status (host/error 404 "nope")) 404)
+(host-hd-test
+  "error ok:false"
+  (contains? (dream-resp-body (host/error 404 "nope")) "\"ok\":false")
+  true)
+(host-hd-test
+  "error message"
+  (contains? (dream-resp-body (host/error 404 "nope")) "\"error\":\"nope\"")
+  true)
+(host-hd-test
+  "error content-type json"
+  (dream-resp-header (host/error 500 "boom") "content-type")
+  "application/json")
+
+;; ── host/json-status ───────────────────────────────────────────────
+(host-hd-test
+  "json-status arbitrary status"
+  (dream-status (host/json-status 418 {:a 1}))
+  418)
+(host-hd-test
+  "json-status encodes body"
+  (contains? (dream-resp-body (host/json-status 200 {:a 1})) "\"a\":1")
+  true)
+
+;; ── host/query-int ─────────────────────────────────────────────────
+(define
+  host-hd-req
+  (fn (target) (dream-request "GET" target {} "")))
+
+(host-hd-test
+  "query-int present"
+  (host/query-int (host-hd-req "/x?limit=5") "limit" 10)
+  5)
+(host-hd-test
+  "query-int absent -> fallback"
+  (host/query-int (host-hd-req "/x") "limit" 10)
+  10)
+
+(define
+  host-hd-tests-run!
+  (fn
+    ()
+    {:total (+ host-hd-pass host-hd-fail)
+     :passed host-hd-pass
+     :failed host-hd-fail
+     :fails host-hd-fails}))