chore: bump to nightly-2025-10-21

leanprover-community-mathlib4-bot · leanprover-community-mathlib4-bot · commit afe13f6e909d · 2025-10-21T10:06:38.000Z
diff --git a/lean-toolchain b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:v4.25.0-rc1
+leanprover/lean4:nightly-2025-10-21
