Fix cond ambiguity: check ALL clauses with cond-scheme?, not just first

The cond special form misclassified Clojure-style as scheme-style when
the first test was a 2-element list like (nil? x) — treating it as a
scheme clause ((test body)) instead of a function call. Define
cond-scheme? using every? to check ALL clauses, fix eval.sx sf-cond and
render.sx eval-cond, rewrite engine.sx parse-time/filter-params as
nested if to avoid the ambiguity, add regression tests across eval/
render/aser specs. 378/378 tests pass.

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

shared/sx/ref/test-eval.sx

@@ -277,6 +277,29 @@
                         false "b"
                         :else "c")))
 
+  (deftest "cond with 2-element predicate as first test"
+    ;; Regression: cond misclassifies Clojure-style as scheme-style when
+    ;; the first test is a 2-element list like (nil? x) or (empty? x).
+    ;; The evaluator checks: is first arg a 2-element list? If yes, treats
+    ;; as scheme-style ((test body) ...) — returning the arg instead of
+    ;; evaluating the predicate call.
+    (assert-equal 0 (cond (nil? nil) 0 :else 1))
+    (assert-equal 1 (cond (nil? "x") 0 :else 1))
+    (assert-equal "empty" (cond (empty? (list)) "empty" :else "not-empty"))
+    (assert-equal "not-empty" (cond (empty? (list 1)) "empty" :else "not-empty"))
+    (assert-equal "yes" (cond (not false) "yes" :else "no"))
+    (assert-equal "no" (cond (not true) "yes" :else "no")))
+
+  (deftest "cond with 2-element predicate and no :else"
+    ;; Same bug, but without :else — this is the worst case because the
+    ;; bootstrapper heuristic also breaks (all clauses are 2-element lists).
+    (assert-equal "found"
+      (cond (nil? nil) "found"
+            (nil? "x") "other"))
+    (assert-equal "b"
+      (cond (nil? "x") "a"
+            (not false) "b")))
+
   (deftest "and"
     (assert-true (and true true))
     (assert-false (and true false))