feat: add Dinic maximum flow algorithm - #3223

Description of Change

Adds a self-contained implementation of Dinic's maximum-flow algorithm.

The implementation includes:

• BFS level-graph construction
• DFS blocking-flow computation
• Residual and reverse edges
• int64_t capacities
• Self-tests for a classic flow network, parallel edges, and an unreachable sink
• Doxygen documentation and complexity analysis

The worst-case time complexity is O(V²E).

Checklist

• Added description of change
• Added file name matches file name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate
• I acknowledge that all my contributions will be made under the project's license

Notes: Added graph/dinic_max_flow.cpp as a reusable educational implementation of Dinic's maximum-flow algorithm.