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/platform_py.py

@@ -774,7 +774,7 @@ PRIMITIVES["last"] = lambda c: c[-1] if c and _b_len(c) > 0 else NIL
 PRIMITIVES["rest"] = lambda c: c[1:] if c else []
 PRIMITIVES["nth"] = lambda c, n: c[n] if c and 0 <= n < _b_len(c) else NIL
 PRIMITIVES["cons"] = lambda x, c: [x] + (c or [])
-PRIMITIVES["append"] = lambda c, x: (c or []) + [x]
+PRIMITIVES["append"] = lambda c, x: (c or []) + (x if isinstance(x, list) else [x])
 PRIMITIVES["chunk-every"] = lambda c, n: [c[i:i+n] for i in _b_range(0, _b_len(c), n)]
 PRIMITIVES["zip-pairs"] = lambda c: [[c[i], c[i+1]] for i in _b_range(_b_len(c)-1)]
 ''',