Fix handler lookup: use eval-expr to resolve handler:ex-* symbols

env-get requires (env-get env name) with two args but dispatch.sx
was calling it with one arg, always returning nil. Now uses
eval-expr + make-symbol to resolve the handler binding in the
current env, with cek-try fallback to nil.

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

sx/sx/handlers/dispatch.sx

@@ -3,11 +3,13 @@
   (fn
     (slug)
     (let
-      ((handler-key (str "handler:ex-" slug)) (hdef (env-get handler-key)))
-      (if
-        (nil? hdef)
-        (error (str "Handler not found: " handler-key))
-        (call-handler hdef)))))
+      ((handler-key (str "handler:ex-" slug)))
+      (let
+        ((hdef (cek-try (fn () (eval-expr (make-symbol handler-key))) (fn (err) nil))))
+        (if
+          (nil? hdef)
+          (error (str "Handler not found: " handler-key))
+          (call-handler hdef))))))
 
 (define
   call-handler