[Good first issue] Add a tiny exactness smoke example for incremental BFS

[sauravsingla]

Goal

Add a tiny self-contained example that demonstrates VeloGraphX's correctness-first contract by comparing maintained BFS distances after an update with a fresh/reference BFS result on the same graph.

This is intentionally not a benchmark. It should be a readable correctness example for newcomers.

Suggested scope

• Add one example under examples/ in either Python or C++.
• Construct a small graph in code.
• Compute BFS from a fixed source.
• Apply a small insertion/deletion batch that changes at least one shortest-path distance.
• Update the maintained BFS state.
• Recompute/reference BFS on the updated graph.
• Assert the two distance vectors match.

Acceptance criteria

• No external data or network access required.
• The update actually changes at least one distance, so the example is meaningful.
• The equality check is active in normal Release execution (do not rely on an assert() that disappears under NDEBUG if using C++).
• Keep output short and deterministic.
• Existing tests continue to pass.