Reference code implementation for Nested SDM estimators

This version of code/reexpress/sdm_model.py is the reference implementation for regression tests of the newer (more efficient) reexpress_sdm python package, as well as the on-device Reexpress two macOS application.

This release is not associated with a new model file for the MCP Server. The model file from release v2.5.0 (reexpress_mcp_server_model_v2_5_0.zip) is compatible.