Expose the ranks kernels, and make sort_intervals' groups keyword-only

[marco-mariotti]

Summary

• Binds natural_rank, lexical_rank and fold_ranks from ruranges-core::ranks (0.1.12). Together with sort_intervals these are the whole of a multi-key interval sort: code each key column to an ascending integer, fold all but the innermost two into a group id, one kernel call, one gather. What stays on the caller's side is what genuinely depends on whether the frame is pandas or polars -- column resolution, numeric dtype coding, the final take.
• The ranking functions take the distinct values of a key column, not the rows, so their cost is per distinct value: 25 chromosome names are free however many rows there are. PyBackedStr borrows each Python string's UTF-8 buffer rather than copying it, and the sort runs with the GIL released. Handing 10 million distinct values across the boundary costs 0.16 s worst case (legacy object-dtype pandas columns; zero for pandas 3's arrow-backed str), against 24.9 s to natural-sort them in Python.
• Breaking: sort_intervals takes groups keyword-only now. All three of starts, ends, groups are same-length integer arrays, so a transposition is undetectable at runtime and produces a plausible-looking wrong answer rather than an error -- pyranges1.RangeFrame.sort_ranges shipped for two releases sorting by End before Start for exactly this reason, putting 97,964 of 100,000 rows in the wrong place on a realistic frame. The same mistake is now a TypeError at the first call.
• README gains a worked example (sort_intervals + natural_rank/lexical_rank/fold_ranks composed for a multi-key sort) and a cheat-sheet row for the new functions.
• 0.1.6 -> 0.1.7.

Test plan

• maturin develop --release, then confirmed the bindings agree with Python's natsort 8.4.0 on 18,507 randomized realistic strings
• Confirmed sort_intervals(starts, ends, groups) (old positional call) now raises TypeError
• Rebuilt and smoke-tested against the published ruranges-core 0.1.12 (no path patch)
• README example runs as written

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