Expose subsystem_dressed_distance to Python

[ciaranra]

Why

subsystem_dressed_distance (dressed distance of a subsystem/gauge code) shipped Rust-only in #415, noted as a follow-up. This adds the Python binding so it is usable alongside the other pecos.qec distance entry points.

What changed

• pecos.qec.subsystem_dressed_distance(num_qubits, stabilizers, gauge_generators, logical_zs, logical_xs, max_weight) — a thin binding over the existing Rust function. It converts the four PauliString lists via the established .to_rust() path, returns DistanceResult | None, and maps SubsystemCodeError to ValueError. It inherits the Rust validation: an ill-formed spec (a logical anticommuting with a stabilizer, an invalid gauge/center split) raises rather than returning a wrong number.
• pecos.qec.DistanceResult — a small result wrapper (distance, min_weight_operator), which did not previously exist as a standalone Python type (the exact stabilizer path surfaces DistanceResult only through its outcome enum). Added with .pyi stubs and __init__ exports.

Verification

• Three Python regression tests: Bacon-Shor 3x3 -> dressed distance 3 (matching the Rust test), None past the weight budget, and a ValueError (matched on "anticommute") for a logical that anticommutes with a stabilizer/gauge.
• cargo check -p pecos-rslib clean; cargo clippy --locked -p pecos-rslib --all-targets -- -D warnings, cargo fmt --all --check, and pre-commit run --all-files all clean.
• Binding exercised end-to-end via just build-debug + the test suite.