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/router.sx

@@ -0,0 +1,75 @@
+;; lib/host/tests/router.sx — host app assembly: health endpoint, group mounting,
+;; 404 fallback.
+
+(define host-rt-pass 0)
+(define host-rt-fail 0)
+(define host-rt-fails (list))
+
+(define
+  host-rt-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! host-rt-pass (+ host-rt-pass 1))
+      (begin
+        (set! host-rt-fail (+ host-rt-fail 1))
+        (append! host-rt-fails {:name name :actual actual :expected expected})))))
+
+(define
+  host-rt-req
+  (fn (method target) (dream-request method target {} "")))
+
+;; An app built from one domain group of two routes.
+(define
+  host-rt-app
+  (host/make-app
+    (list
+      (list
+        (dream-get "/ping" (fn (req) (host/ok "pong")))
+        (dream-get "/widgets/:id" (fn (req) (host/ok (dream-param req "id"))))))))
+
+;; ── health ─────────────────────────────────────────────────────────
+(host-rt-test
+  "health status 200"
+  (dream-status (host-rt-app (host-rt-req "GET" "/health")))
+  200)
+(host-rt-test
+  "health body healthy"
+  (contains?
+    (dream-resp-body (host-rt-app (host-rt-req "GET" "/health")))
+    "healthy")
+  true)
+
+;; ── group routes mounted ───────────────────────────────────────────
+(host-rt-test
+  "group route ping"
+  (contains?
+    (dream-resp-body (host-rt-app (host-rt-req "GET" "/ping")))
+    "pong")
+  true)
+(host-rt-test
+  "group path param"
+  (contains?
+    (dream-resp-body (host-rt-app (host-rt-req "GET" "/widgets/42")))
+    "\"data\":\"42\"")
+  true)
+
+;; ── fallback ───────────────────────────────────────────────────────
+(host-rt-test
+  "unknown path 404"
+  (dream-status (host-rt-app (host-rt-req "GET" "/nope")))
+  404)
+(host-rt-test
+  "wrong method 405"
+  (dream-status (host-rt-app (host-rt-req "POST" "/ping")))
+  405)
+
+(define
+  host-rt-tests-run!
+  (fn
+    ()
+    {:total (+ host-rt-pass host-rt-fail)
+     :passed host-rt-pass
+     :failed host-rt-fail
+     :fails host-rt-fails}))