Updates the Stan submodule to e667e0a79.

stan-buildbot · stan-buildbot · commit 93aec3b3d3f6 · 2026-06-02T15:07:46.000-04:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit 3884f60c54261c9b3a51a6147a6d3f6d2a7060c6
+Subproject commit e667e0a79231f508e434c3271f5d74e44870591c
