Test runner: return-value error handling, no guard/cek-try/throws

guard and cek-try both create CEK frames that don't survive async
perform/resume. Instead, run-action returns nil on success and an
error string on failure. The for-each loop checks the return value
and sets fail-msg. No exceptions cross async boundaries.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/static/wasm/sx/hs-parser.sx

@@ -1859,7 +1859,8 @@
                   (let
                     ((acc2 (append acc (list cmd))))
                     (cond
-                      ((match-kw "then") (cl-collect acc2))
+                      ((match-kw "then")
+                        (cl-collect (append acc2 (list (quote __then__)))))
                       ((and (not (at-end?)) (= (tp-type) "keyword") (cmd-kw? (tp-val)))
                         (cl-collect acc2))
                       (true acc2)))))))
@@ -1868,7 +1869,10 @@
             (cond
               ((= (len cmds) 0) nil)
               ((= (len cmds) 1) (first cmds))
-              (true (cons (quote do) cmds))))))
+              (true
+                (cons
+                  (quote do)
+                  (filter (fn (c) (not (= c (quote __then__)))) cmds)))))))
       (define
         parse-on-feat
         (fn