Validate XOR checksum on all incoming BLE commands

[skialpine]

This is untested, but rather then just report a bug I thought I would propose a fix.

Summary

• Move validateChecksum() call to the top of the BLE command dispatcher in onWrite()
• All incoming commands are now rejected if the XOR checksum doesn't match
• Remove the tare-only log-only validation (which logged but didn't actually reject bad packets)

Problem

The existing code has validateChecksum() and calculateChecksum() methods, but they were only used for the tare command (0x0F) — and even then, the result was only logged, not enforced. The tare executed regardless of checksum validity.

All other commands — including power off (0x0A/0x02), system reset (0x1F), and calibration (0x1A) — were processed without any checksum verification.

Fix

A single validateChecksum() check at the top of the data[0] == 0x03 block, with early return on failure. This protects all commands with one check.

Risk

Low. BLE already has link-layer CRC, so corrupted packets reaching the application are rare. This is defense-in-depth using the existing validation function. The only behavioral change is that corrupted packets are now silently dropped instead of executed.

Testing

Found while writing unit tests for Decenza scale protocol parsing. Cross-referenced the openscale firmware to verify checksum behavior.

[tadelv]

Thanks for the PR @skialpine ! Looks neat.
Have you tested this with any app other than Decenza as well?

[skialpine]

This is untested as I had issues building the firmware, but as I said above rather than just post an issue I thought I would propose a solution.

[tadelv]

@Sofronio please take a look. I'm worried this would potentially break other client apps that do not care about the checksum byte, not including it in the payload.

@skialpine thanks again - but I have my reservations as written above. From what I remember, the checksum byte is not really used and should be ignored on both sides - a discrepancy between the documentation and reality. But I could be wrong.

[skialpine]

You are correct, this would break clients that do not send checksums. I will look at how important it is and get back to you.