feat: add Strand Sort and fix sort_utils unused import warnings

[AliAlimohammadi]

Description

Implements Strand Sort, a comparison-based sorting algorithm that works by repeatedly extracting increasing subsequences ("strands") from the input and merging them into a growing result list.

Algorithm:

1. Remove the first element of the remaining input and start a new strand.
2. Scan the rest of the input left-to-right; whenever an element is ≥ the last element of the strand, pull it out and append it to the strand. One full pass yields one sorted strand.
3. Merge the strand into the accumulated result via a standard two-way merge.
4. Repeat until the input is empty.

Also fixes pre-existing unused import warnings in sort_utils.rs by gating use rand::RngExt and use std::time::Instant with #[cfg(test)], matching the functions that use them.

Type of change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue) — unused import warnings in sort_utils.rs

Checklist

• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked CONTRIBUTING.md and my code follows its guidelines.

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.91304% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.00%. Comparing base (2cb9392) to head (d98845c).

Files with missing lines	Patch %	Lines
src/sorting/strand_sort.rs	98.91%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1036   +/-   ##
=======================================
  Coverage   95.99%   96.00%           
=======================================
  Files         391      392    +1     
  Lines       29630    29722   +92     
=======================================
+ Hits        28444    28535   +91     
- Misses       1186     1187    +1     

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[siriak]

Looks good, thanks!