CHANGES-202605.md: add entries for #424, #425, #426

[thegushi]

Summary

Follows the pattern established in #423's changelog entry -- adds one entry each for the three static-analysis fix PRs merged from the same cppcheck pass:

• Security: Fix out-of-bounds read / size_t underflow in dkimf_db_datasplit() #425 (LMDB dkimf_db_datasplit out-of-bounds read / size_t underflow)
• Correctness fixes: Fix _Bool functions that silently discard -1 error returns #424 (_Bool functions silently discarding -1 error returns)
• Code quality and cleanup: Static analysis cleanup: dead code, format string, size_t nits #426 (grab-bag of dead code / format string / size_t nits)

No code changes, documentation only.