Missing array index OOB check

### What is wrong?

See the following code:

https://github.com/ethereum/fe/blob/7054185bae601702dc6b15ecf752d8810222a9fd/crates/tests/fixtures/files/arrays.fe#L68-L102

### How can it be fixed

Add a compile-time check for array indices.

Start with crates/tests/fixtures/files/arrays.fe at lines 68-102, then trace how the compiler handles the array indexing shown there. Confirm the expected behavior for an out-of-bounds constant index and add coverage for the compile-time diagnostic. Done means invalid array indices are rejected during compilation while valid indices continue to compile.

Written by the indexing model from the issue text.