53b8b6e282
Every CI job used dtolnay/rust-toolchain on stable, so the declared minimum supported Rust version was never exercised — an accidental use of a newer API would only break for downstream users on an older compiler. Add an `msrv` job with a two-row matrix: the workspace crates (wickra-core, wickra, wickra-data) build and test on Rust 1.75, and the node binding on Rust 1.77, matching the rust-version each manifest declares. Both rows use the SHA-pinned toolchain action.