feat(algorithms): add counting sort

[Robertboy18]

So I also wanted to add the counting sort for List ℕ, using an explicit upper bound on the values in the input! The key idea is that it scans the input once for each key from 0 through the bound, counts how many times that key occurs, and emits that many copies. The cost model counts one comparison for each inspected input element during these scans.

The main correctness proof is by counts: every value appears in the output exactly as many times as it appeared in the input. From there, I just prove that sortedness, permutation correctness, stability via StableByValue, and the exact running time in this cost model is all satisfied: )!

Validation:

• lake build Cslib

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a formal counting sort for List ℕ (with TimeM cost model) plus shared “stability by value” utilities, and re-exports them from the root module.

Changes:

• Introduce StableByValue predicate for list outputs to preserve per-value subsequences.
• Add countingSort implementation returning TimeM ℕ (List ℕ) with correctness + time complexity theorems.
• Export the new modules from Cslib.lean.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
Cslib/Algorithms/Lean/Sorting.lean	Adds a reusable stability-by-value predicate for sorting utilities.
Cslib/Algorithms/Lean/CountingSort/CountingSort.lean	Implements counting sort in TimeM and proves correctness/time bounds.
Cslib.lean	Re-exports the new CountingSort and Sorting modules.