host: discoverable log in / log out footer link

Login had no visible entry point — you could only reach it by hitting a guard.
Add an auth footer the pages splice in: "log in" when logged out, "signed in
as <user> · log out" when logged in.

- host/auth-footer: SX fragment reading the session principal; guards a
  session-less request so it's safe to call anywhere.
- GET /logout added alongside POST so the footer link is a plain <a> (logout
  is low-harm; GET is acceptable). Clears the session, redirects home.
- home and post pages splice (host/auth-footer req) into their footer.

Tests: home + post footers show a login link when anonymous; GET /logout ->
303. 221/221. Verified live: anonymous shows "log in"; logged in shows
"signed in as admin · log out"; /logout reverts it.

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

lib/host/tests/blog.sx

@@ -203,6 +203,14 @@
 (host-bl-test "edit missing post -> 404"
   (dream-status (host-bl-wapp (host-bl-send "GET" "/ghost/edit" "Bearer good" "" ""))) 404)
 
+;; -- auth footer (discoverable login/logout) --
+(host-bl-test "home footer shows a log in link when anonymous"
+  (contains? (dream-resp-body (host-bl-app (host-bl-req "/"))) ">log in</a>") true)
+(host-bl-test "post footer shows a log in link when anonymous"
+  (contains? (dream-resp-body (host-bl-app (host-bl-req "/my-first-post/"))) ">log in</a>") true)
+(host-bl-test "GET /logout -> 303"
+  (dream-status (host-bl-app (host-bl-req "/logout"))) 303)
+
 ;; -- experimental unguarded create-only route (POST /new, no auth) --
 (define host-bl-oapp (host/make-app (list host/blog-open-create-routes host/blog-routes)))
 (host/blog-use-store! (persist/open))