Add greedy algorithm implementations

Implement classic greedy algorithms and problems:

## Algorithms to implement:
- Activity selection problem
- Huffman coding
- Fractional knapsack
- Minimum spanning tree (Prim's and Kruskal's)
- Dijkstra's shortest path
- Job sequencing with deadlines
- Egyptian fractions
- Interval scheduling

## Requirements:
- Include proof of correctness for each algorithm
- Provide detailed docstrings with complexity analysis
- Implement test cases with varied input scenarios
- Add example use cases and applications

## Educational content:
- Explain the greedy choice property
- Contrast with dynamic programming approaches
- Document cases where greedy algorithms fail
- Show greedy algorithm design patterns

This collection will demonstrate important optimization techniques and trade-offs in algorithm design.

No files, tests, or entry points are named; first review the repository's existing algorithm implementations and testing conventions. Done means implementing all listed greedy algorithms, with correctness proofs, complexity-focused docstrings, varied tests, examples, and the requested educational explanations.

Written by the indexing model from the issue text.