Add cargo-semver-checks to Rust CI workflow (#1875)

This adds semver validation to catch breaking changes before release.
The check runs on Ubuntu during CI and compares against the published crate on crates.io.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: haixuanTao

.github/workflows/rust.yml

@@ -102,3 +102,9 @@ jobs:
         shell: bash
         working-directory: ./tokenizers
         run: cargo readme > must_match_readme.md && diff must_match_readme.md README.md
+
+      - name: Check semver
+        if: matrix.os == 'ubuntu-latest'
+        uses: obi1kenobi/cargo-semver-checks-action@v2
+        with:
+          manifest-path: ./tokenizers/Cargo.toml