publish-crate: external-consumer check fails on rustc 1.94 (traverse-contracts 0.16.1 needs 1.95)

The v0.31.0 publish succeeded on crates.io but the post-publish isolated external-consumer step failed: it resolves traverse-contracts 0.16.1 (requires rustc 1.95) while .github/workflows/publish-crate.yml pins toolchain 1.94.0. Every future release will fail the same way. Fix: bump the publish workflow toolchain to 1.95.0. Governing spec: 010-crate-publish-pipeline.