Issue 5.4: Implement Network State Query API

[0xMimir]

Build HTTP API endpoints for querying network state during test execution, allowing external monitoring tools to observe test progress. Endpoints: GET /nodes (list all nodes with status), GET /nodes/{id} (specific node details), GET /network/state (aggregated network state), GET /network/tips (best tips by node), GET /metrics/gossip (gossip consistency metrics), GET /test/status (current test progress). Use axum for HTTP server, start on configurable port, shut down cleanly with test.