Validate payload reads and entrypoint bounds

[ericlewis]

Summary

• Check header and payload reads for exact lengths.
• Reject empty, too large, or overflowed payload sizes.
• Validate the entry point is inside the loaded payload.
• Use fixed-width integer and address-sized math.

Why

payload.bin can be missing, cut short, or changed. The loader used to trust the size and entry point from that file. A bad file could make the loader read into the wrong memory or jump outside the payload.

Verification

• python3 generate_start.py
• cmake -S . -B /tmp/playbrew-verify-harden-payload-load
• cmake --build /tmp/playbrew-verify-harden-payload-load
• git diff --check