host: wire :body into live rendering — composition fold is fold #1, live (roadmap step 2)

A record may carry a :body (a composition node); host/blog-post renders it via the
render-fold (host/comp-render) against a context built from the principal (auth), else the
legacy sx_content path. compose.sx loaded into the host (serve.sh + conformance.sh module
lists). host/blog-body-of / host/blog--set-body!.

Seeded /compose-demo: ONE composition object that shows seq + alt(when auth) + row(par) +
each, and renders DIFFERENTLY by context. Verified live-path (ephemeral SX_SERVING_JIT=1):
anon -> login-prompt (else) + columns + event list; authed -> member block (when auth),
login-prompt gone. The object is the program; the render is the execution -- now live.
Focused eval confirms the in-process render matches the test (ANON<span>..> vs MEMBER<..>).
Tests added; full blog suite still box-contended.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/serve.sh

@@ -86,6 +86,7 @@ MODULES=(
   "lib/host/sx/kg-cards.sx"
   "lib/host/feed.sx"
   "lib/host/relations.sx"
+  "lib/host/compose.sx"
   "lib/host/blog.sx"
   "lib/host/server.sx"
 )
@@ -152,6 +153,11 @@ EPOCH=1
   echo "(epoch $EPOCH)"
   echo "(eval \"(host/blog-seed-types!)\")"
   EPOCH=$((EPOCH+1))
+  # Seed a live demo of the composition fold (plans/composition-objects.md): /compose-demo
+  # is one composition object rendered by host/comp-render — renders differently by context.
+  echo "(epoch $EPOCH)"
+  echo "(eval \"(host/blog-seed-compose-demo!)\")"
+  EPOCH=$((EPOCH+1))
   # Load relation metadata (symmetry/labels) from the relation-posts into the
   # in-memory cache, so render paths read it without a (VmSuspending) durable read.
   echo "(epoch $EPOCH)"