feat: add Skip List (Data-Structures/Linked-List)

[ChrisJr404]

What

Adds a Skip List implementation under Data-Structures/Linked-List/SkipList.js with a companion test file under the existing test/ directory.

A skip list is a probabilistic ordered-set / map data structure with expected O(log n) search, insert and delete. Each newly inserted node is promoted to the next level with probability p (default 1/2), giving an expected height of log_{1/p}(n). Compared to balanced BSTs (AVL, red-black) it is much simpler to implement because there are no rotations - balance is maintained statistically rather than structurally. Its semantics also map cleanly onto a multi-level linked list, which is why it lives next to the other linked-list variants in this repo.

Reference: https://en.wikipedia.org/wiki/Skip_list

API

SkipList exposes:

• insert(key, value?) (returns this, chainable; updates value if key exists)
• search(key) -> value or undefined
• has(key) -> boolean
• delete(key) -> boolean
• size() / isEmpty()
• entries() generator yielding [key, value] pairs in ascending key order
• Symbol.iterator yielding keys in ascending order

The constructor accepts { maxLevel, p, random } so the RNG can be injected for deterministic testing.

Tests

Data-Structures/Linked-List/test/SkipList.test.js adds 14 cases covering:

• empty / size / isEmpty
• insert + search (with and without explicit value)
• key reinsertion updates value without growing size
• has() membership
• delete present / delete absent
• ascending iteration regardless of insertion order
• entries() output
• string keys with lexicographic ordering
• a 500-op randomized stress workload (deterministic LCG) cross-checked against a Set reference
• input validation on the constructor (maxLevel, p, random)
• behaviour under forced-min and forced-max promotion levels

Checks

• npm run check-style clean
• npm test -> 347 files / 11984 tests pass locally (including the 14 new ones)

DIRECTORY.md is intentionally not touched in this PR - the UpdateDirectory workflow will regenerate it on push.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.35691% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.12%. Comparing base (5c39e87) to head (a21e687).

Files with missing lines	Patch %	Lines
Data-Structures/Linked-List/SkipList.js	99.35%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1896      +/-   ##
==========================================
+ Coverage   85.91%   86.12%   +0.20%     
==========================================
  Files         379      380       +1     
  Lines       19778    20089     +311     
  Branches     3016     3068      +52     
==========================================
+ Hits        16993    17302     +309     
- Misses       2785     2787       +2     

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