fix: resolve review findings and release 1.9.1

[wlaur]

Summary

Resolve every actionable finding in the holistic 1.9.0 review and prepare patch release 1.9.1.

Correctness and consistency

• mask every plugin row with a non-finite CoolProp input, including IF97 trivial/state-independent outputs
• treat a single-species bracket fraction as an absolute concentration only for INCOMP; HEOS::CO2[0.5] now raises, while [1.0] remains accepted
• require fractions for multi-species names instead of degrading missing mixture configuration to NaN
• validate Polars Series magnitudes as numeric, normalize integer/empty Series to Float64, and reject bool/string/list/datetime/unit-typed Series
• refuse implicit absolute-temperature ↔ temperature-difference reinterpretation when reading QuantityFrame data
• propagate unsupported scalar CoolProp input pairs consistently with array/expression paths
• make floor division use divide-then-floor semantics across float, numpy, Series, and Expr magnitudes
• reject mixed vector-container arithmetic before numpy/pint/polars dispatch can become state-dependent
• reject Expr → Series astype instead of creating an Object Series
• share Quantity unit spelling normalization with the Polars dtype layer and turn malformed tokenizer failures into UndefinedUnitError

Native/plugin and maintenance

• enable i8/i16/u8/u16 Polars features so small-integer inputs cannot panic the generated wrapper
• reject nonnumeric plugin columns instead of non-strictly casting bool/string values
• retain scalar states after a failed flash
• range-check percent-form INCOMP concentrations
• use a checked c_long conversion for input pairs on Windows
• tighten installed-wheel docs-root discovery
• refresh stale docs/output/size/license wording and remove the vestigial macOS 10.15 override (arm64 correctly defaults to 11.0)
• bump pyproject.toml and uv.lock to 1.9.1

Compatibility note

This deliberately tightens accepted inputs: CoolProp's high-level parser silently ignores a single-species HEOS fraction, while its low-level state API can compute with the unnormalised vector. encomp now consistently requires non-INCOMP fractions to sum to one, so HEOS::CO2[0.5] raises instead of returning silently wrong physics.

Local validation

• ruff check / ruff format --check
• pyright strict, pyrefly, ty: zero errors
• cargo fmt --check, clippy -D warnings, Rust tests: pass
• full default suite without Python CoolProp: 1050 passed, 2 expected oracle skips
• explicit CoolProp 8.0.0 oracle parity: 100 passed
• local and Read the Docs-parity Sphinx builds with warnings as errors: pass
• locally built encomp-1.9.1-cp313-abi3-macosx_11_0_arm64.whl installed in a fresh external Python 3.14 env against current PyPI dependencies (including Polars 1.43), native self-check passed, no Python CoolProp installed, shipped suite: 797 passed, 20 expected skips