Credence.Rule.NoSortThenAtUnfixable (credence v0.3.1)

Performance rule (flag-only companion): Detects Enum.sort |> Enum.at(index) patterns where the index is not a compile-time literal 0 or -1, or the sort direction cannot be statically determined.

These cases require human or LLM judgement to rewrite because the correct replacement depends on the runtime value of the index and/or sort direction.

Examples that trigger this rule

Enum.sort(nums, :desc) |> Enum.at(k - 1)       # variable index
Enum.at(Enum.sort(nums), mid)                   # variable index
Enum.sort(nums, dir) |> Enum.at(0)              # variable direction
Enum.sort(nums, fn a, b -> a > b end) |> Enum.at(0)  # custom comparator

What to do

For index 0 → Enum.min/1 or Enum.max/1. For index -1 → Enum.max/1 or Enum.min/1 (reversed). For other indices → Enum.take/2 on the sorted list, or a partial-sort / quickselect algorithm.