Add payload build system

[ericlewis]

Summary

• adds a tested Python packager that writes the PlayBrew /payload.bin header
• builds a hello payload through CMake as hello-payload.elf, hello-payload.raw, and hello-payload.bin
• runs Python tests in the release workflow, packages hello-payload.bin, and documents payload building and troubleshooting

Why

People should not have to hand-make payload.bin. This adds a small build path that creates the file the loader expects, like using a recipe instead of guessing the ingredients.

Verification

• python3 -m unittest discover -s tests
• python3 -m py_compile combine.py generate_start.py firmware_addresses.py tools/package_payload.py tests/test_combine.py tests/test_package_payload.py
• cmake -S . -B /tmp/playbrew-payload-system-final -G Ninja && cmake --build /tmp/playbrew-payload-system-final
• checked hello-payload.bin header: size=87, entry=0, total=95
• parsed .github/workflows/release.yml as YAML
• git diff --check HEAD~2..HEAD