Fix: replace guard with cek-try in test runner, clear stale reuse_stack

The guard form (call/cc + handler-bind expansion) doesn't survive async
IO suspension — the CEK continuation from guard's call/cc captures frames
that become invalid after the VM resumes from hs-wait. Replacing guard
with cek-try (which compiles to VM-native OP_PUSH_HANDLER/OP_POP_HANDLER)
avoids the CEK boundary crossing.

The test runner now executes: suspends on hs-wait, resumes, runs test
actions, and test assertions fire correctly. The "Not callable: nil"
error is eliminated. Remaining: test assertion errors from iframe content
not loading fast enough (timing issue, not a framework bug).

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

sx/sx/applications/htmx/runner.sx

@@ -105,13 +105,11 @@
                   (reset! current (str "Running: " name))
                   (reset! results (assoc (deref results) name "running"))
                   (reload-frame)
-                  (guard
-                    (e
-                      (true
-                        (reset! results (assoc (deref results) name "fail"))
-                        (console-log (str "FAIL " name ": " e))))
-                    (for-each run-action (get test :actions))
-                    (reset! results (assoc (deref results) name "pass")))))
+                  (let
+                    ((test-ok (cek-try (fn () (for-each run-action (get test :actions)) true) (fn (e) (do (reset! results (assoc (deref results) name "fail")) (console-log (str "FAIL " name ": " e)) false)))))
+                    (when
+                      test-ok
+                      (reset! results (assoc (deref results) name "pass"))))))
               tests)
             (reset! running false)
             (reset! current "Done"))))