Fix defhandler blockers: &key params, multi-body, HTML tags

Three fixes in run_tests.ml defhandler parser:
1. Extract &key param names, store in hdef["params"]. run-handler
   binds them from mock args before evaluating — fixes row-editing,
   tabs, inline-edit, profile-editing handlers.
2. Capture all body forms after params, wrap in (do ...) when
   multiple — fixes ex-slow (sleep before let).
3. Register all HTML tags as native fns via Sx_render.html_tags —
   fixes ex-bulk (tr tag), and enables aser to serialize any tag.

1352 → 1361 passing tests.

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

web/tests/test-swap-integration.sx

@@ -84,12 +84,24 @@
   run-handler
   (fn
     (hdef)
-    (let
-      ((result (aser (get hdef "body") (get hdef "closure"))))
-      (if
-        (sx-expr? result)
-        (sx-expr-source result)
-        (if (string? result) result (str result))))))
+    (do
+      (let
+        ((params (get hdef "params")) (closure (get hdef "closure")))
+        (when
+          (and params (not (empty? params)))
+          (for-each
+            (fn
+              (p)
+              (let
+                ((val (get _mock-args p)))
+                (env-bind! closure p (or val nil))))
+            params)))
+      (let
+        ((result (aser (get hdef "body") (get hdef "closure"))))
+        (if
+          (sx-expr? result)
+          (sx-expr-source result)
+          (if (string? result) result (str result)))))))
 
 (defsuite
   "swap:click-to-load"