Updates the Stan submodule to 838a88d99.

stan-buildbot · stan-buildbot · commit f81b69ccecde · 2025-07-02T04:48:16.000-04:00
diff --git a/stan b/stan
@@ -1 +1 @@
-Subproject commit 066164d3e3692a2852540fb87da50f2b9578b735
+Subproject commit 838a88d99a878cb3f19662ddac4e07f183bd332a
