Pin the toolchain to 1.98.1 so the fork checks the way the consumer builds it

rust-toolchain.toml

@@ -0,0 +1,11 @@
+# Pinned so a fresh clone (and `cargo install --git`, which picks rustc by CWD)
+# builds with exactly the compiler this workspace was verified against.
+[toolchain]
+channel = "1.98.1"
+components = ["rustfmt", "clippy"]
+targets = [
+    "wasm32-unknown-unknown",
+    "aarch64-linux-android",
+    "x86_64-linux-android",
+    "aarch64-apple-ios",
+]