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/eval.sx

@@ -309,14 +309,18 @@
         nil))))
 
 
+;; cond-scheme? — check if ALL clauses are 2-element lists (scheme-style).
+;; Checking only the first arg is ambiguous — (nil? x) is a 2-element
+;; function call, not a scheme clause ((test body)).
+(define cond-scheme?
+  (fn (clauses)
+    (every? (fn (c) (and (= (type-of c) "list") (= (len c) 2)))
+            clauses)))
+
 (define sf-cond
   (fn (args env)
-    ;; Detect scheme-style: first arg is a 2-element list
-    (if (and (= (type-of (first args)) "list")
-             (= (len (first args)) 2))
-      ;; Scheme-style: ((test body) ...)
+    (if (cond-scheme? args)
       (sf-cond-scheme args env)
-      ;; Clojure-style: test body test body ...
       (sf-cond-clojure args env))))
 
 (define sf-cond-scheme