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

<!-- Copyright (c) 2026 bentzn --> <!-- SPDX-License-Identifier: Apache-2.0 --> # raposza-model Project-owned domain types: the LF and contract semantic core. No SDK types. No dependency on any target module, and none on `raposza-spi` either - the direction is one way, and that is what the split is