ci: check semver compatibility

[brndnmtthws]

Summary

Add cargo-semver-checks to the normal build-and-test workflow so API compatibility is checked before release tags are created.

User Impact

SemVer-incompatible public API changes will fail regular pull request CI according to the version already declared in Cargo.toml, giving maintainers feedback before the tag-triggered publish workflow runs.

Root Cause

The existing CI validates builds, tests, features, MSRV, documentation, and dependencies, but it does not compare the current public API with the latest published crate. The first release-specific validation currently happens after a tag is pushed.

Fix

Add a standalone semver compatibility job that installs cargo-semver-checks with the stable Rust toolchain and runs cargo semver-checks --verbose on pushes to main and pull requests targeting main.

Validation

• mise x actionlint@1.7.12 shellcheck@0.11.0 -- actionlint .github/workflows/build-and-test.yml
• git diff --check HEAD^
• cargo semver-checks --verbose against published dryoc 0.9.0 and current 1.0.0

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.47%. Comparing base (a603c72) to head (12a5c1b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #147   +/-   ##
=======================================
  Coverage   82.47%   82.47%           
=======================================
  Files          58       58           
  Lines        4427     4427           
=======================================
  Hits         3651     3651           
  Misses        776      776           

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