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/feed.sx

@@ -0,0 +1,22 @@
+;; lib/host/feed.sx — Feed domain endpoints on the host. The first real endpoint
+;; migrated onto the SX host: the activity timeline, read straight from the feed
+;; subsystem's public API (feed/all + the stream combinators) and serialised as
+;; JSON. GET /feed returns recent-first activities; ?actor=<id> filters by actor
+;; and ?limit=<n> caps the count. Depends on lib/feed/* + lib/host/handler.sx.
+
+;; GET /feed -> recent-first activities as a JSON envelope.
+;; Query: ?actor=<id> (filter) ?limit=<n> (cap, applied after filtering).
+(define host/feed-timeline
+  (fn (req)
+    (let ((base (feed/recent (feed/all)))
+          (actor (dream-query-param req "actor")))
+      (let ((filtered (if actor (feed/by-actor base actor) base))
+            (limit (dream-query-param req "limit")))
+        (let ((capped
+                (if limit (feed/take filtered (string->number limit)) filtered)))
+          (host/ok (feed/items capped)))))))
+
+;; Route group contributed by the feed domain.
+(define host/feed-routes
+  (list
+    (dream-get "/feed" host/feed-timeline)))