Type provider steppers with the Array protocol

[jc-macdonald]

Summary

• type flepimop2-op_system flat stepper state and results with Flepimop2's backend-neutral Array protocol
• retain the existing namespace-preserving implementation with no runtime conversion
• assert that the JIT path returns a native jax.Array and namespace
• point development checks at the reviewed Flepimop2 callable-contract commit from #348
• document the provider contract and update the shared changelog

Closes #229.

Validation

• just ci: Ruff and strict mypy green; 527 core tests passed with 14 skips; 69 provider tests passed with 1 optional skip; strict docs green
• just build-all-provider: wheel metadata passed and clean Python 3.12 wheel tests passed against published Flepimop2 0.3.0