fix(spammer): reject --mix weights whose total overflows u32

[kutluhaneth46]

Summary

Fixes #352: --mix accepted individually valid u32 weights whose sum overflows u32::MAX (e.g. transfer=4294967295,legacy=1), then panicked in TxTypeMix::total_weight() during Config::validate().

Changes

• Add TxTypeMix::checked_total_weight() using checked addition
• Reject overflowing totals in FromStr with a clear --mix error
• Make Config::validate() return an error instead of panicking
• Keep total_weight() for post-validation callers (expects a safe total)
• Tests: parser reject/accept boundaries, validate no-panic, CLI try_parse_from

Test plan

• cargo test -p spammer overflowing
• cargo test -p spammer tx_type_mix
• Confirm ordinary mixes (transfer=70,legacy=30) still parse

[huklaa]

I claimed #352 at 13:09 PDT with the exact implementation scope stated publicly. This PR was then opened about 13 minutes later and covers that same scope. Opening a duplicate implementation after a visible claim, without first coordinating on the issue, is not consistent with fair and collaborative open-source contribution etiquette. Please withdraw/close #355 and allow me to complete the work I claimed first. If you had independently started before my claim, please provide that chronology so maintainers can assess it transparently.