select CI toolchains explicitly

[danieleades]

Summary

• remove the repository-level rust-toolchain.toml pin;
• select the intended Rust toolchain explicitly in each CI job;
• test stable on Linux, macOS, and Windows;
• add a nightly Linux test without changing unrelated action versions.

Relationship to #938

This is stacked on #938, which declares and checks the project's MSRV. The current diff therefore includes #938 until that PR merges; after it lands, this PR should be rebased onto main and will contain only the toolchain-policy and CI-matrix changes.

rust-toolchain.toml is useful for pinning a contributor toolchain, but it does not declare or enforce the minimum Rust version supported by downstream consumers. #938 establishes that contract. This follow-up removes the contributor pin and makes each CI toolchain choice explicit instead.

Validation

• formatting and workspace tests pass;
• the MSRV check from add MSRV check to CI #938 passes on Rust 1.88;
• stable CI covers Linux, macOS, and Windows;
• nightly CI covers Linux.