* chore: bump Rust toolchain to 1.96
* ci: add cargo-public-api breaking-change check
Adds a release-type-aware `cargo-public-api` diff job that fails on
changed/removed public items unless the PR's commits mark a breaking
(major) release. This catches source-breaking API changes that
cargo-semver-checks cannot yet detect (e.g. function return-type or
field-type changes).
