Support custom step sizes

It would be nice to be able to configure custom step sizes.
For example, one might want to access array elements like this:
```c
int a[] = { 1, 2, 3 };
printf("%d %d %d", a[2.1], a[2.2], a[2.3]);
// Expected output: 1 2 3
```

The issue names no files, tests, or entry points. Start by locating the compiler's array-indexing implementation and understand how expressions such as a[2.1] are parsed and evaluated. Done should include a defined custom-step-size behavior matching the example and coverage for the resulting output.

Written by the indexing model from the issue text.