chore(deps): remove graph-expansion package (moved to graphbox)

The graph-expansion package has been migrated to the graphbox repository.
This package provided graph traversal and expansion algorithms.

Removed:
- BFS/DFS traversal implementations
- Bidirectional BFS
- Ego network extraction
- Degree-prioritised expansion
- Baseline traversal algorithms (random, frontier-balanced, standard BFS)
- Graph expander interfaces

Author: Mearman