Add introspective sort implementations

[BMNTR]

Closes #73.

Summary

• Added a Python implementation of introspective sort that starts with quicksort, falls back to heapsort at the recursion-depth limit, and uses insertion sort for small partitions.
• Added a Java implementation with the same quicksort/heapsort/insertion-sort hybrid behavior and a small runnable demo.

Validation

• python sorting/introspective_sort/introspective_sort.py
• python -m py_compile sorting/introspective_sort/introspective_sort.py
• git diff --check

Note: Java compilation was not run locally because this environment does not have a JDK installed.

Implemented with assistance from OpenAI Codex.