Phase-3 runtime capability and lifecycle foundation

[grape-wzy]

Summary

• add the Phase-3 runtime capability model and deterministic -MN_ENOTSUP handling
• introduce mn_system_init() lifecycle flow with private silicon/BSP hooks
• add host and STM32 silicon/BSP scaffolds plus a host reference device path
• extend host tests and Phase-3 documentation while keeping make test and STM32 build validation working

Validation

• make test
• cmake --build --preset host-debug
• ctest --preset host-debug
• cmake --preset stm32f407-debug
• cmake --build --preset stm32f407-debug
• git diff --check