Keystone 3 firmware

This commit makes two clean-up changes to Tron transaction handling in the Keystone 3 firmware. First, it avoids calling the same formatting functions twice by reusing computed values. Second, it improves how the signing code handles the wallet's secret seed: it now initializes the seed buffer to the correct size, checks whether retrieving the seed succeeded before using it, and securely wipes the seed from memory even if signing fails. These are defensive hardening improvements rather than a fix for a known active exploit, but they reduce the risk that a failed seed retrieval or leftover seed data could lead to incorrect signing or information leakage.

This commit is a cleanup and hardening pass on how the Keystone 3 hardware wallet handles camera images, SD card files, and debug/test code. The most user-relevant change is that file-reading functions now refuse to load very large files from the SD card, which closes a path where an oversized file could exhaust device memory. Several debug helpers that could print sensitive file contents or cryptographic hashes over the serial port, plus a leftover firmware-copy routine, are removed. A camera preview buffer is also made safer by freeing it on re-init instead of leaking memory. The commit message is vague ('review data flow') and does not call this a security fix, so the security relevance is inferred from the code changes rather than stated by the vendor.

This commit only adds new automated tests for the Tron cryptocurrency transaction handling code in a hardware wallet firmware project. It does not change any production behavior, fix bugs, or introduce new features. There is no security-relevant change visible in the diff.