[leninworld]

This PR fixes issue #426.

Rust’s ApproxSchurConfig::split now uses NonZeroU32, so zero cannot be constructed or deserialized. Python stores the same invariant while keeping its integer API, defaults, read-only getter and existing ValueError message. Positive serialized values retain their encoding; existing binary fixtures are unchanged. The Rust field-type change is documented as breaking under Unreleased.

Validation:

• Full Docker validation: 573 Rust tests/doctests passed, one existing ignored; rebuilt Python suite passed; 97.28% line coverage. The baseline fails the zero-deserialization regression; the candidate passes, including nine positive-wire cases, both type examples, existing fixtures, pickle and solver reuse.
• Formatting, warnings-denied Clippy, MSRV 1.85, deny, semver against 0.4, release wheel/sdist, installed-wheel API demo and benchmark smoke passed.
• Ruff 0.9.10 lint and formatting checks passed on both changed Python files. A single assertion-wrap adjustment after the full run is AST-identical to the tested file; the Ruff run checks those final bytes.

The Docker runs use native Linux ARM64/Python 3.12. They do not claim the upstream x64/free-threaded, macOS, Windows or complete release-wheel matrix. If #482 lands first, its new factor_config conversion site will need .get() too.

Recorded Docker validation summaries, not fresh terminal captures:

Prepared with AI assistance.

Closes #426.