build: pin the Rust toolchain instead of floating on stable

Two CI failures in a row were caused by dtolnay/rust-toolchain@stable
picking up a newer Rust the moment it shipped, surfacing new
clippy/rustc lints unrelated to whatever change was actually being
pushed — hard to tell "my code broke this" from "the toolchain moved."

Add rust-toolchain.toml (channel 1.98.0, the version CI had already
moved to) so rustup resolves the same toolchain locally automatically
— no more local/CI drift — and point build.yml/verify.yml/release.yml
at that exact version instead of the floating @stable ref. Bumping the
pin is now a deliberate, single-purpose commit instead of a surprise.

Local toolchain updated to 1.98.0 and full `cargo xtask verify` passes
clean on it.