Expand graph, stream, reactive, and state-machine toolkit

[takeokunn]

Summary

• Adds a large batch of graph algorithms (traversal orders, centrality/distance metrics, dominators, max-flow/min-cut, coloring, clustering, Eulerian paths, and more) plus export/serialization/diff/layout utilities
• Adds reactive subjects (push-based dual of the lazy streams) with full operator parity, and rounds out stream/pipeline/state-machine/effects ergonomics (iteration, builders, execution, introspection, observability)
• Adds an end-to-end integration example composing pipelines, graphs, streams, reactive, state machines, and serialization

Test plan

• nix flake check passes locally (tests, coverage, paredit-lint)
• CI green on GitHub Actions (ubuntu-latest / macos-latest)