fix(spammer): reject overflowing transaction mix totals

[huklaa]

Summary

• make "TxTypeMix" aggregate-weight handling overflow-safe
• reject invalid "--mix" totals at the parser/configuration boundary instead of panicking
• add focused regression coverage for overflowing totals

Fixes #352

Context

I claimed #352 first and prepared this branch shortly afterward. A later duplicate PR was closed in favor of this earlier claim, so this PR continues that original work.

Validation

• "cargo fmt --check" — passed
• "cargo test -p spammer" — passed (75 library tests, 10 binary tests, 0 failures)
• "cargo clippy -p spammer --all-targets -- -D warnings" — passed with 0 warnings