feat(python): bind pairwise streams

[maxi-maxima]

Closes #882

What changed

• add a hand-written PyStream::pairwise binding that keeps the previous PyElement as resettable engine state
• emit a real Python 2-tuple from the second tick onward, so pairwise().split() composes without another conversion path
• expose the fluent Python Stream.pairwise() method and document it beside difference()
• add Rust seam and pytest regressions using non-arithmetic string values; both pin the quiet first tick and the split round trip

Why

The Rust pairwise op already reaches all three engines, but its typed (T, T) output cannot pass through the ordinary erased #[pyop] route as one PyElement. The Python binding therefore needs the same state machine at the erased edge and must explicitly materialize a Python tuple.

Validation

• cargo fmt --all -- --check
• cargo metadata --no-deps --format-version 1
• git diff --check
• python -m py_compile crates/wingfoil-python/tests/test_interop.py
• checked the tuple construction against the vendored PyO3 0.29.1 API

The Rust lint/test commands and maturin develop && pytest cannot start on this Windows host because the MSVC linker (link.exe) is not installed. The PR is draft so the upstream Linux checks can provide the executable validation before it is marked ready.