fix: handle math.gcd int64 minimum

[vycdev2]

Summary

• compute math.gcd with unsigned magnitudes so int64 minimum inputs work when the result is representable
• report a deterministic overflow error when the mathematical GCD is 2^63 and cannot fit int64
• add interpreter and runtime-failure regression coverage

Verification

• cargo test -q -p jett_comptime builtin_math_gcd — passed (2 tests)
• cargo test -q -p jett_driver --test fixture_suite math_gcd_reports_unrepresentable_result — passed (1 test)
• cargo build — passed
• cargo test -q — passed (full workspace suite)
• cargo fmt --check — passed
• cargo run -q -p jett_cli -- format --check tests/runtime_fail/math_gcd_int64_min.jett — passed
• git diff --check — passed

Risk

• Low: the change is limited to math.gcd integer magnitude handling and adds boundary regressions.

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

[vycdev]

The unsigned-magnitude GCD fix is correct in isolation and its focused tests passed, but it now conflicts with the merged math.lcm work from #83 in the interpreter and fixture suite. Please rebase, preserve both checked-math implementations and regressions, then rerun the comptime/driver tests.