c_runtime: WKT override-hook passthrough (ra6 3k) A plan whose descriptor carries desc.encode / desc.decode now dispatches through those overrides instead of erroring. Top-level encode/decode, sub-message fields, and map<,message> values all check the override refs and call them with the same contract the pure-Lua codec uses: encode(value) -> body bytes; decode(buf) -> value. runtime/pb/wkt.lua is unmodified. ra6 3k