host: live writes via signed sessions + kernel multi-Set-Cookie (193/193)

Unblock the guarded blog write routes for browsers: a login form sets a
signed session cookie that the same routes accept (alongside Bearer), so
publishing works end-to-end on blog.rose-ash.com without Quart.

- kernel: http-listen emit serialises a response :set-cookies LIST as one
  Set-Cookie header each (a headers dict can't hold more than one). Purely
  additive — responses without :set-cookies are unchanged.
- server.sx: host/-dream->native forwards :set-cookies to the native resp.
- lib/host/session.sx: durable, signed sessions on the persist KV
  (session/create|exists|get|set|clear), wired via dream-sessions-signed.
- lib/host/auth.sx: GET/POST /login + POST /logout; host/require-user accepts
  a session principal OR a Bearer token.
- router.sx: host/make-app wraps the whole app in the session middleware and
  auto-mounts /login + /logout — the front door always has sessions.
- blog.sx: write routes use host/require-user; serve.sh flips POST /new from
  the experimental UNGUARDED route to the guarded write routes, with admin
  creds + signing secret + ACL grant from the container env.
- session conformance suite (12): login->cookie->guarded write 201; no
  cookie/forged/logged-out -> 401; Bearer fallback still works.

Verified live on blog.rose-ash.com: 401 unauthenticated, 303 login, 303
publish, anonymous read renders, post persists across container recreate.

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

lib/host/blog.sx

@@ -234,7 +234,7 @@
     (host/pipeline
       (list
         host/wrap-errors
-        (host/require-auth resolve)
+        (host/require-user resolve)
         (host/require-permission "edit" (fn (req) "blog")))
       h)))
 (define host/blog-write-routes