Fix quasiquote flattening bug, decouple relations from evaluator

- Fix qq-expand in eval.sx: use concat+list instead of append to prevent
  nested lists from being flattened during quasiquote expansion
- Update append primitive to match spec ("if x is list, concatenate")
- Rebuild sx_ref.py with quasiquote fix
- Make relations.py self-contained: parse defrelation AST directly
  without depending on the evaluator (25/25 tests pass)
- Replace hand-written JSEmitter with js.sx self-hosting bootstrapper
- Guard server-only tests in test-eval.sx with runtime check

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/test-eval.sx

@@ -545,9 +545,12 @@
 
 
 ;; --------------------------------------------------------------------------
-;; defpage
+;; Server-only tests — skip in browser (defpage, streaming functions)
+;; These require forms.sx which is only loaded server-side.
 ;; --------------------------------------------------------------------------
 
+(when (get (try-call (fn () stream-chunk-id)) "ok")
+
 (defsuite "defpage"
   (deftest "basic defpage returns page-def"
     (let ((p (defpage test-basic :path "/test" :auth :public :content (div "hello"))))
@@ -716,3 +719,5 @@
                :content (~chunk :val val))))
       (assert-equal true (get p "stream"))
       (assert-true (not (nil? (get p "shell")))))))
+
+) ;; end (when has-server-forms?)