145: Gateway bridge + internet connectivity + CLI commands

[forkwright]

Summary

• GatewayBridge (bridge.rs): multi-gateway selection by priority and health rank, automatic failover with 30s cooldown, Healthy → Degraded → Offline transitions at 5s/20% loss thresholds
• MQTT parsing (mqtt.rs): decode_service_envelope, decode_map_report, decode_proxy_message; extracts NodeNum from !hexid gateway ID strings
• MeshCollector run loop (collector.rs): full implementation wiring transport connect, config handshake, PacketProcessor (topology + GeoSignals), discovery manager, heartbeat, gateway health monitor, router outbound flush, and graceful CancellationToken shutdown
• Mesh CLI (mesh/mod.rs): akroasis mesh status, nodes, send, topology with comfy-table formatting; live-data helpers build_status_table / build_nodes_table for daemon integration
• Example config (docs/akroasis-mesh.toml): annotated mesh section for ~/.config/akroasis/config.toml covering serial, TCP, PSK, store-and-forward, topology, and gateway node settings

Test coverage

238 tests across kerykeion + akroasis:

• bridge.rs: 11 tests (gateway selection, failover, health transitions, health monitor cancellation)
• mqtt.rs: 8 tests (ServiceEnvelope, MapReport, proxy message roundtrips, gateway ID parsing)
• collector.rs: 13 tests (probe, packet processing, hop count, router state, flush cancellation)
• mesh/mod.rs: 15 tests (node ID parsing, table formatting, dispatch, gateway health display)

Validation

cargo fmt --all -- --check   ✓
cargo clippy --workspace --all-targets -- -D warnings   ✓
cargo test --workspace   ✓  549 passed

Observations

• The akroasis serve daemon mode remains a stub — full collector lifecycle (start/stop, config reload) will be wired when daemon is implemented
• Discovery's run_discovery dispatches traceroutes via the primary connection only; multi-connection traceroute fan-out is deferred
• format_gateway_health retains a wildcard arm for forward-compatibility with #[non_exhaustive] GatewayHealth additions
• Open PRs feat(kerykeion): node discovery, mesh topology graph, and peer tracking #68 (discovery) and 144: Message routing + store-and-forward + delivery confirmation #69 (routing) represent work that was squash-merged to main in commits 570ef34 and 8a7d306 before this branch; those PRs can be closed as stale

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