[OTHER] Refactor FlatGraph

[aturker-synnada]

Summary

The current implementation of FlatGraph uses a nested structure with separate Node and GConnection classes, creating unnecessary complexity and indirection. This makes the code harder to understand, maintain, and debug. It also impacts performance.

Context

Current FlatGraph implementation has drawbacks:

• More difficult to debug and understand the code flow
• Reduced performance for large graphs
• Extra complexity in the graph traversal code