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/engine.sx

@@ -34,11 +34,12 @@
 (define parse-time
   (fn (s)
     ;; Parse time string: "2s" → 2000, "500ms" → 500
-    (cond
-      (nil? s) 0
-      (ends-with? s "ms") (parse-int s 0)
-      (ends-with? s "s")  (* (parse-int (replace s "s" "") 0) 1000)
-      :else (parse-int s 0))))
+    ;; Uses nested if (not cond) because cond misclassifies 2-element
+    ;; function calls like (nil? s) as scheme-style ((test body)) clauses.
+    (if (nil? s) 0
+      (if (ends-with? s "ms") (parse-int s 0)
+        (if (ends-with? s "s") (* (parse-int (replace s "s" "") 0) 1000)
+          (parse-int s 0))))))
 
 
 (define parse-trigger-spec
@@ -219,20 +220,19 @@
     ;; Filter form parameters by sx-params spec.
     ;; all-params is a list of (key value) pairs.
     ;; Returns filtered list of (key value) pairs.
-    (cond
-      (nil? params-spec) all-params
-      (= params-spec "none") (list)
-      (= params-spec "*") all-params
-      (starts-with? params-spec "not ")
-        (let ((excluded (map trim (split (slice params-spec 4) ","))))
-          (filter
-            (fn (p) (not (contains? excluded (first p))))
-            all-params))
-      :else
-        (let ((allowed (map trim (split params-spec ","))))
-          (filter
-            (fn (p) (contains? allowed (first p)))
-            all-params)))))
+    ;; Uses nested if (not cond) — see parse-time comment.
+    (if (nil? params-spec) all-params
+      (if (= params-spec "none") (list)
+        (if (= params-spec "*") all-params
+          (if (starts-with? params-spec "not ")
+            (let ((excluded (map trim (split (slice params-spec 4) ","))))
+              (filter
+                (fn (p) (not (contains? excluded (first p))))
+                all-params))
+            (let ((allowed (map trim (split params-spec ","))))
+              (filter
+                (fn (p) (contains? allowed (first p)))
+                all-params))))))))
 
 
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