spec: match special form — ADT constructor pattern matching (20 tests)

Extends match-pattern in spec/evaluator.sx with an ADT case: when the
pattern is (CtorName var...) and the value is an ADT dict (:_adt true),
check :_ctor matches, arity matches, then recursively bind field patterns.
Supports nested patterns, wildcard _, variable binding, and zero-arg ctors.

Changes step-sf-match to route no-clause errors through raise-eval-frame
instead of direct error, allowing guard to catch non-exhaustive matches.

40/40 ADT tests pass (20 define-type + 20 match). Zero regressions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/evaluator.sx

@@ -2736,6 +2736,17 @@
       (= value (nth pattern 1))
       (symbol? pattern)
       (do (env-bind! env (symbol-name pattern) value) true)
+      (and (list? pattern) (not (empty? pattern)) (symbol? (first pattern)) (dict? value) (get value :_adt))
+      (let
+        ((ctor-name (symbol-name (first pattern)))
+          (field-patterns (rest pattern))
+          (fields (get value :_fields)))
+        (and
+          (= (get value :_ctor) ctor-name)
+          (= (len field-patterns) (len fields))
+          (every?
+            (fn (pair) (match-pattern (first pair) (nth pair 1) env))
+            (zip field-patterns fields))))
       (and (dict? pattern) (dict? value))
       (every?
         (fn (k) (match-pattern (get pattern k) (get value k) env))
@@ -2780,7 +2791,7 @@
         ((result (match-find-clause val clauses env)))
         (if
           (nil? result)
-          (error (str "match: no clause matched " (inspect val)))
+          (make-cek-value (str "match: no clause matched " (inspect val)) env (kont-push (make-raise-eval-frame env false) kont))
           (make-cek-state (nth result 1) (first result) kont))))))
 
 (define