fix: report math.sum overflow

[vycdev2]

Summary

• replace unchecked math.sum accumulation with checked int64 addition
• return a deterministic runtime error instead of panicking or wrapping on overflow
• add a runtime-failure regression for int64.MAX + 1

Verification

• cargo test -q -p jett_driver math_sum_reports_overflow — passed (1 test)
• cargo build — passed
• cargo test -q — passed
• cargo fmt --check — passed
• git diff --check — passed

Risk

• Low: the change is limited to the existing math.sum overflow path; representable sums and existing type diagnostics are unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.