Occaisional corruption of the downloaded hex file --> The hex file cannot be decoded. Checksum calculation failure occurred

This is a tricky to track down issue, so hopefully posting it here we'll find other people with the same problem...

Though we haven't ever seen or reproduced it ourselves in the foundation, a number of people have sent us hex files that are missing large chunks of data, and the result is that the checksums fail and DAPLink refuses (correctly!) to flash the file.

The error DAPLink puts in the fail.txt is:
The hex file cannot be decoded. Checksum calculation failure occurred

Missing chunks are not on line boundaries.

No source files or tests are named. Start by reproducing the browser's downloaded .hex file and comparing it with the expected file, then inspect DAPLink's fail.txt output and the missing non-line-boundary chunks. Done means identifying the corruption path, preventing truncated downloads, and confirming the resulting file passes checksum validation.

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