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-render.sx

@@ -151,6 +151,13 @@
     (assert-equal "<p>hello</p>"
       (render-html "(let ((x \"hello\")) (p x))")))
 
+  (deftest "cond with 2-element predicate test"
+    ;; Regression: cond misclassifies (nil? x) as scheme-style clause.
+    (assert-equal "<p>yes</p>"
+      (render-html "(cond (nil? nil) (p \"yes\") :else (p \"no\"))"))
+    (assert-equal "<p>no</p>"
+      (render-html "(cond (nil? \"x\") (p \"yes\") :else (p \"no\"))")))
+
   (deftest "let preserves outer scope bindings"
     ;; Regression: process-bindings must preserve parent env scope chain.
     ;; Using merge() on Env objects returns empty dict (Env is not dict subclass).