feat: add spine-skip-server-start guard for testing

spine.el

@@ -43,6 +43,10 @@ Returns the rendered string."
      (buffer-string))
    context))
 
+(defvar spine-skip-server-start nil
+  "When non-nil, do not start the HTTP server.
+Set by test harnesses that only need the model functions.")
+
 ;; --- handlers ---------------------------------------------------------
 
 (defservlet hello text/html ()
@@ -51,11 +55,12 @@ Returns the rendered string."
                 ("message" "Hello from Emacs.")))))
 
 ;; --- start ------------------------------------------------------------
-(setq httpd-host spine-host)
-(setq httpd-port spine-port)
-(httpd-start)
-(message "Spine listening on http://%s:%d"
-         (or spine-host "localhost") spine-port)
+(unless spine-skip-server-start
+  (setq httpd-host spine-host)
+  (setq httpd-port spine-port)
+  (httpd-start)
+  (message "Spine listening on http://%s:%d"
+           (or spine-host "localhost") spine-port))
 
 (provide 'spine)
 ;;; spine.el ends here