Updates the Stan submodule to a78a30eca.

stan-buildbot · stan-buildbot · commit d7fc91999c9f · 2026-02-13T21:29:28.000-05:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit 06d7216fb391542a320343d1af61e365c9227d28
+Subproject commit a78a30eca98e80307ff453e37dfb4b3028ec2fd9
