Aether SunSpec extension

Optional compile-time SunSpec model catalog and point-expansion support for AetherEdge Modbus compositions.

The default AetherEdge runtime does not compile this crate. Composition roots opt in through the aether-io/sunspec feature. The extension contains no transport implementation; discovery uses the Modbus adapter selected by aether-io.

Vendored model JSON and its upstream Apache-2.0 license are under models/.