Add automated release + crates.io publish pipeline

[user1303836]

Summary

• add PR-time release checks requiring version bump + changelog entry
• add post-merge publish workflow for crates.io, tags, and GitHub releases
• add helper scripts for Cargo metadata parsing and changelog section extraction
• document setup and release flow in RELEASING.md and README

Included automation

• .github/workflows/release-check.yml
• .github/workflows/publish-crate.yml
• .github/scripts/cargo-package-field.sh
• .github/scripts/changelog-section.sh
• RELEASING.md

Notes

• publish workflow is idempotent (skips publish when crate version already exists)
• requires CARGO_REGISTRY_TOKEN secret in repository settings