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/render.sx

@@ -134,12 +134,8 @@
 ;; (test body test body ...).
 (define eval-cond
   (fn (clauses env)
-    (if (and (not (empty? clauses))
-             (= (type-of (first clauses)) "list")
-             (= (len (first clauses)) 2))
-      ;; Scheme-style
+    (if (cond-scheme? clauses)
       (eval-cond-scheme clauses env)
-      ;; Clojure-style
       (eval-cond-clojure clauses env))))
 
 (define eval-cond-scheme