feat(kerykeion): mesh topology graph and node management

From Phase 03 PLAN.md, REQ-04.

Real-time mesh network topology:
- Graph of all 6 mesh nodes + gateway
- Per-node: status, SNR, hop counts, routes
- `akroasis mesh status` CLI showing topology
- `akroasis mesh send` and `akroasis mesh nodes` commands
- Position and telemetry data from mesh nodes

Success criterion: all 6 mesh nodes + gateway managed via `akroasis mesh status`.