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/handler.sx

@@ -0,0 +1,39 @@
+;; lib/host/handler.sx — Host handler layer: the bridge from a Dream request to a
+;; subsystem call and back to a Dream response. A host handler IS a Dream handler
+;; (request -> response); these helpers build the JSON envelope every host
+;; endpoint shares: {"ok":true,"data":...} on success, {"ok":false,"error":...}
+;; on failure. Plus a status-carrying JSON constructor that Dream's own dream-json
+;; (200-only) lacks, and a couple of request-reading conveniences.
+;; Depends on lib/dream/types.sx + lib/dream/json.sx.
+
+;; ── responses ──────────────────────────────────────────────────────
+
+;; JSON response at an arbitrary status (dream-json is 200-only).
+(define host/json-status
+  (fn (status value)
+    (dream-response status {:content-type "application/json"}
+      (dream-json-encode value))))
+
+;; Success envelope: 200 {"ok":true,"data":<value>}.
+(define host/ok
+  (fn (value)
+    (host/json-status 200 {:ok true :data value})))
+
+;; Success envelope at a chosen status (e.g. 201 for a created resource).
+(define host/ok-status
+  (fn (status value)
+    (host/json-status status {:ok true :data value})))
+
+;; Error envelope: {"ok":false,"error":<message>} at the given status.
+(define host/error
+  (fn (status message)
+    (host/json-status status {:ok false :error message})))
+
+;; ── request reading ────────────────────────────────────────────────
+
+;; Integer query param with a fallback (query params arrive as strings).
+;; Absent param -> fallback; present -> parsed number.
+(define host/query-int
+  (fn (req name fallback)
+    (let ((raw (dream-query-param req name)))
+      (if raw (string->number raw) fallback))))