Set up Zephyr native_sim test target with ztest framework

[flwrenn]

Summary

Set up a host-native test framework using Unity v2.6.1 and CTest so firmware algorithm modules can be compiled and tested on the development machine without hardware.

Note: The original plan was Zephyr ztest + native_sim, but native_sim only targets Linux (POSIX architecture) and does not work on macOS. Unity + CTest was chosen instead — it provides the same coverage for pure-C algorithm modules and runs on any host platform.

Area

Tooling / CI

Done when

• tests/ directory exists with a CMake project that fetches Unity v2.6.1 via FetchContent
• Algorithm sources are compiled into a hallmark_algorithms static library (explicit source listing, no file(GLOB))
• A trivial smoke test suite builds and passes on the host machine
• Tests can be run via cmake -B build/tests tests/ && cmake --build build/tests && ctest --test-dir build/tests

Dependencies

None

Context

All algorithm modules (rapid trigger, HRM, SOCD) need a host-side test harness. This must be done before any algorithm implementation so tests can be written alongside code.