sort: full `-k KEYDEF` parsing with start/end fields, character positions, and per-key flags

[chaliy]

Problem

sort -k currently only parses a single field number (e.g. -k2 or -k2n). The GNU coreutils key specification is much richer and scripts in the wild rely on it.

Missing features

• End-field specs: -k2,3 (sort by fields 2 through 3)
• Character positions: -k2.3,3.4 (start at field 2 char 3, end at field 3 char 4)
• Per-key flags: -k2n,2 vs -k3r,3 (different sort modes per key)
• Multiple keys: -k1,1 -k2n (primary sort by field 1 lexically, secondary by field 2 numerically)

Current code

sortuniq.rs:137-145 — parses -k value by extracting leading digits and checking for n/r suffix. No comma handling, no multi-key support.

Suggested approach

uutils/coreutils (MIT) has a robust key parser in their sort implementation that handles the full GNU syntax. Consider borrowing or adapting the parsing logic. The comparison/sorting itself can stay as-is since it already works against bashkit's VFS.

References

• GNU sort key spec: https://www.gnu.org/software/coreutils/manual/html_node/sort-invocation.html
• uutils sort: https://github.com/uutils/coreutils/tree/main/src/uu/sort