[ImportVerilog] String casts to packed byte arrays fail

Nobody has claimed this yet.

`circt-verilog --import-only` rejects a string cast to a multidimensional packed integral type, including inside an uncalled function:

```systemverilog
module top;
typedef bit [255:0][7:0] bytes_t;
function automatic bytes_t convert(string value);
return bytes_t'(value);
endfunction
endmodule
```

Run `circt-verilog --import-only --top=top string_cast.sv -o /dev/null`.

The importer reports:

```text
error: unsupported conversion from '!moore.string' to '!moore.array<256 x i8>'
```

This legal cast should import with the usual string packing, zero-padding, and truncation semantics. Both two-state and four-state packed arrays should be supported.

Assisted-by: Codex:GPT-6

Start by reproducing the issue with circt-verilog --import-only using the shown string_cast.sv example and trace the importer’s unsupported conversion diagnostic. Done means the legal string cast imports successfully with string packing, zero-padding, and truncation for both two-state and four-state packed arrays.

Written by the indexing model from the issue text.