Define loader writable data startup contract

[ericlewis]

Summary

• Add explicit .data and .bss sections to the loader linker script.
• Add linker assertions that fail if writable loader state appears without startup support.
• Move alignment so an empty .data section does not create fake bytes.

Why

This loader starts without the normal C startup code. That means no one copies .data or clears .bss before it runs. If a future change adds global writable data, it could act wrong on the Playdate. This makes the build stop until startup support exists.

Verification

• python3 generate_start.py
• cmake -S . -B /tmp/playbrew-verify-loader-runtime-contract
• cmake --build /tmp/playbrew-verify-loader-runtime-contract
• arm-none-eabi-objdump -h /tmp/playbrew-verify-loader-runtime-contract/loaderRevA
• git diff --check