fix(bash): negative array indexing ${arr[-1]}

[chaliy]

Summary

• Fix ${arr[-1]} to return last element instead of first
• Fix arr[-1]=val to assign to last element instead of first
• Negative i64 indices are now resolved as array_length + index
• 4 new test cases: last, second-to-last, first-via-negative, write-negative

Test plan

• 4 new tests covering read and write with negative indices
• cargo test --all-features passes
• cargo clippy and cargo fmt clean