raposza/tools: platform/raposza-spi/README.md

<!-- Copyright (c) 2026 bentzn --> <!-- SPDX-License-Identifier: Apache-2.0 --> # raposza-spi The translation service-provider interface, and the machine-readable capability declarations. `LedgerClient_i` and `LfDecoder_i` live here because a translation target implements them. `raposza-model` holds