fix: report list.sum integer overflow

[vycdev2]

Summary

• Use checked addition for integer list.sum evaluation so overflow returns a Jett runtime error instead of panicking in the host interpreter.
• Add a runtime-failure fixture covering the overflow boundary.

Verification

• cargo fmt --all -- --check — passed.
• cargo check -q — passed.
• cargo build -q — passed.
• cargo test -q -p jett_driver list_sum_reports_overflow -- --nocapture — passed.
• cargo test -q -p jett_driver run_pass_list_higher_order -- --nocapture — passed.
• cargo test -q — passed (full workspace, including 433 driver fixtures).
• git diff --check — passed.

Risk

• Low: the change only replaces an integer-overflow panic with the existing runtime-error path; successful sums and mixed-type handling are unchanged.

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

[vycdev]

The checked list.sum accumulation is correct in isolation and its regressions passed, but this branch now conflicts with current main in the shared interpreter/fixture area. Please rebase, preserve all already-merged runtime-failure tests, and rerun comptime/driver tests.