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/bootstrap_py.py

@@ -539,6 +539,9 @@ class PyEmitter:
         clauses = expr[1:]
         if not clauses:
             return "NIL"
+        # Check ALL clauses are 2-element lists (scheme-style).
+        # Checking only the first is ambiguous — (nil? x) is a 2-element
+        # function call, not a scheme clause ((test body)).
         is_scheme = (
             all(isinstance(c, list) and len(c) == 2 for c in clauses)
             and not any(isinstance(c, Keyword) for c in clauses)