Improve payload read diagnostics

[ericlewis]

Summary

• retries short payload reads until the requested header or body bytes are loaded
• logs the failed payload section, byte progress, last sd_read result, decoded header, load address, and entry address
• validates entry offsets more clearly, including Thumb alignment and enough remaining bytes for one instruction

Why

If /payload.bin fails to load, the old message only said the payload could not be read. This makes the loader say what went wrong in plain terms, so debugging is closer to seeing which page is missing instead of only knowing the whole book failed.

Verification

• python3 -m unittest discover -s tests
• python3 -m py_compile combine.py generate_start.py firmware_addresses.py tests/test_combine.py
• cmake -S . -B /tmp/playbrew-payload-read-build -G Ninja && cmake --build /tmp/playbrew-payload-read-build
• git diff --check