release: encomp 1.9.0

[wlaur]

Summary

Release encomp 1.9.0 after #21:

• removes the Python CoolProp runtime package and uses one bundled native CoolProp implementation
• simplifies scalar and Polars fluid-property evaluation around that single backend
• adds independent test-only CoolProp parity coverage, packaging checks, and performance benchmarks
• updates the README and documentation for the simplified API

Validation

• implementation CI: https://github.com/wlaur/encomp/actions/runs/29842004810
• this PR changes only the version in pyproject.toml and uv.lock

After this PR is green and merged, tag v1.9.0 to publish PyPI and create the GitHub release.