test(router): add unit tests and relay routing integration test

[Sendi0011]

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Title: test(router): add unit tests and relay routing integration test

Closes: #47

Summary

Adds external test coverage for PathFinder and RelayRouter, and an end-to-end relay routing integration test as specified in Issue #47. Also exposes the router module publicly in src/lib.rs, which was the only missing wiring preventing external tests from compiling.

Changes

src/lib.rs — added pub mod router;

tests/router_test.rs — 9 unit tests:

• test_pathfinder_ranks_relay_adjacent_peer_first — 1-hop peer sorts before 5-hop peer regardless of input order
• test_pathfinder_handles_empty_connections — empty input returns empty, no panic
• test_pathfinder_unknown_peers_ranked_last — peers with no HopCounter entry sort to the end
• test_pathfinder_follows_graph_edges_to_relay — PathFinder traverses MeshGraph edges to find relay proximity
• test_relay_router_respects_target_fanout — 10 peers, fanout=3 → exactly 3 returned
• test_relay_router_falls_back_to_random_when_no_relay_path — all peers at 255 hops → still returns target_fanout peers
• test_relay_router_returns_all_when_fanout_exceeds_peer_count
• test_relay_router_empty_input_returns_empty
• test_relay_router_zero_fanout_returns_empty

tests/integration/relay_routing_test.rs — 3 integration tests:

• test_message_routes_to_relay_node — 4-node chain (Node0–Node1–Node2–Node3/Relay), message injected at Node0 reaches the relay ✓
• test_message_injected_at_relay_is_received
• test_isolated_node_message_stays_at_origin

Note on MeshSimulator (#20): Since Issue #20 is not yet merged, a minimal MeshBuilder harness is implemented inline in the integration test using the same PathFinder + RelayRouter + MeshGraph + HopCounter stack. When #20 lands, the harness can be swapped for the canonical MeshSimulator with no changes to the test assertions.

Test results

router_test:        9 passed, 0 failed
relay_routing_test: 3 passed, 0 failed

## Proof

<img width="1136" height="880" alt="Screenshot 2026-03-10 at 06 18 50" src="https://github.com/user-attachments/assets/daedca04-34dc-4464-b0bf-9595b16aa8c3" />

<img width="1136" height="880" alt="Screenshot 2026-03-10 at 06 19 01" src="https://github.com/user-attachments/assets/a26d26c7-adf6-44b4-839c-15b94f062386" />

Closes #47 

[Sendi0011]

GM @Mrwicks00 this trust you are doing great? this pr is ready for review.