`create_netclass` previously called legacy NETCLASSES.Find/.Add APIs that
were removed in KiCad 10. NETCLASS getters like GetMicroViaDiameter that
also no longer exist crashed any subsequent edit. The schema accepted
`traceWidth` but the handler only read `trackWidth`, so requests using
the documented field silently produced no-op netclasses.
- Add a KiCad-version-defensive shim around netclass creation that
prefers the new netclasses_map dict-style API and falls back to legacy.
- Introduce _safe_get/_safe_set helpers so missing getters/setters on
KiCad 10 NETCLASS objects fail gracefully instead of raising.
- Accept both traceWidth and trackWidth in the request payload.
Net classes still need to be written into .kicad_pro directly because
KiCad 10 stores them in net_settings.classes and the MCP only writes
to .kicad_pcb; that's a separate fix.
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