fix: report math.factorial overflow

[vycdev2]

Summary

• replace saturating factorial multiplication with checked multiplication
• report the exact overflowing operation instead of returning int64::MAX
• add a runtime-fail regression for math.factorial(21)

Verification

• cargo test -q -p jett_driver math_factorial_reports_overflow — passed (1 test)
• cargo build — passed
• cargo test -q — passed
• cargo fmt --check — passed
• cargo run -q -p jett_cli -- format --check tests/runtime_fail/math_factorial_overflow.jett — passed
• git diff --check — passed

Risk

• Low: the change is limited to the overflow path for one integer math builtin; representable and negative inputs retain their existing paths.

Closes #105

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

[vycdev]

The checked-factorial implementation is correct in isolation and its focused regression passed, but this branch now conflicts with current main in the shared interpreter/fixture area. Please rebase, preserve all already-merged checked-math tests, and rerun the focused and full workspace suites.