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/eval.sx

@@ -656,8 +656,8 @@
                   (let ((spliced (trampoline (eval-expr (nth item 1) env))))
                     (if (= (type-of spliced) "list")
                       (concat result spliced)
-                      (if (nil? spliced) result (append result spliced))))
-                  (append result (qq-expand item env))))
+                      (if (nil? spliced) result (concat result (list spliced)))))
+                  (concat result (list (qq-expand item env)))))
               (list)
               template)))))))