Use dom-on for event handlers; add CI config and stepper Playwright test

- web/orchestration.sx, web/signals.sx: dom-listen → dom-on (trampoline
  wrapper that resolves TCO thunks from Lambda event handlers)
- .gitea/: CI workflow and Dockerfile for automated test runs
- tests/playwright/stepper.spec.js: stepper widget smoke test
- Remove stale artdag .pyc file

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

web/orchestration.sx

@@ -1107,7 +1107,7 @@
                     (mark-processed! el (str "on:" event-name))
                     ;; Parse body as SX, bind handler that evaluates it
                     (let ((exprs (sx-parse body)))
-                      (dom-listen el event-name
+                      (dom-on el event-name
                         (fn (e)
                           (let ((handler-env (env-extend (dict))))
                             (env-bind! handler-env "event" e)
@@ -1211,7 +1211,7 @@
             (mark-processed! el "emit")
             (let ((event-name (dom-get-attr el "data-sx-emit")))
               (when event-name
-                (dom-listen el "click"
+                (dom-on el "click"
                   (fn (e)
                     (let ((detail-json (dom-get-attr el "data-sx-emit-detail"))
                           (detail (if detail-json (json-parse detail-json) (dict))))

web/signals.sx

@@ -425,7 +425,7 @@
 
 (define on-event :effects [io]
   (fn (el (event-name :as string) (handler :as lambda))
-    (dom-listen el event-name handler)))
+    (dom-on el event-name handler)))
 
 ;; Convenience: create an effect that listens for a DOM event on an
 ;; element and writes the event detail (or a transformed value) into
@@ -436,7 +436,7 @@
 (define bridge-event :effects [mutation io]
   (fn (el (event-name :as string) (target-signal :as signal) transform-fn)
     (effect (fn ()
-      (let ((remove (dom-listen el event-name
+      (let ((remove (dom-on el event-name
               (fn (e)
                 (let ((detail (event-detail e))
                       (new-val (if transform-fn