[codekongdev]

Summary

Add a Python solution for HackerRank's Count Triplets as solutions/python/practice409.py. It counts earlier single values and pairs waiting for each next value, preserving index order in expected O(n) time and O(n) space. Completing triplets before registering the current value handles a ratio of 1 without reusing an index.

Includes 16 executable assertions covering the three official samples, duplicate values, short inputs, index order, large values, and a 100,000-element input.

Related issue

Closes #158

Validation

• python3 solutions/python/practice409.py — all 16 assertions passed.
• Compared against an independent brute-force oracle for 16,380 exhaustive cases and 1,000 seeded random cases; input lists remain unchanged.
• Required solution-header validation passed for the added file.
• Syntax and whitespace checks passed.
• HackerRank platform submission and hidden tests have not been run; these results are from local validation.

Checklist

• The solution includes the required problem, source, and note header comment.
• The file is in the correct solutions/<language>/ folder and follows the practice<n> naming convention.
• I ran the solution and checked that it works.
• I have kept this pull request focused on one change.