add MSRV check to CI

[danieleades]

Summary

• declare Rust 1.88 as the workspace MSRV;
• apply that declaration to all four published packages;
• add an additive CI job that checks each published package at its declared MSRV;
• enable Serde's derive feature for the build dependency exposed by --no-dev-deps.

This PR deliberately keeps rust-toolchain.toml, the existing build-and-test matrix and job name, and current action versions unchanged. The contributor-toolchain and broader stable/nightly CI policy is split into #1058.

Validation

cargo hack --workspace --exclude typify-test --exclude example-build --exclude example-macro --no-dev-deps --rust-version check
cargo +1.89.0 fmt --all -- --check
cargo +1.89.0 test --workspace --locked

All commands pass locally; the MSRV checks select Rust 1.88 from the declared package metadata.

[danieleades]

@ahl is this of interest?