[ImportVerilog] Streaming operators fail on unpacked arrays

Streaming concatenation operators (`{>>{...}}`, `{<<{...}}`) need to convert non-packed, fixed-size aggregates (unpacked arrays, unpacked structs) to and from a flat packed bit vector, and ImportVerilog has no such conversion, so any streaming operand that isn't already a plain packed type fails to import. This bug used to be hidden by the `moore.conversion` op that pretended to convert an unpacked array to a dense bit vector. That op no longer exists after #11024.

```systemverilog
module Streaming;
logic [15:0] vec_3;
logic [31:0] vec_1;
logic arr_1 [63:0];

initial begin
vec_1 = {<' to '!moore.l16'
```

Per IEEE 1800-2017 §11.4.14, streaming braces accept packed types, strings, unpacked arrays (fixed-size, dynamic, and queues), unpacked structs/unions, and class handles (source-only); `with [..]` part-select is further restricted to fixed-size unpacked arrays, dynamic arrays, and queues.

`Context::materializeConversion` in ImportVerilog needs a conversion path for fixed-size aggregates, gated on both sides having equal, statically-known `UnpackedType::getBitSize()`. `moore.conversion`'s existing MooreToCore lowering already does a width-checked bitcast for such pairs, so this is mostly a frontend-side gap. Queues, dynamic arrays, and associative arrays should stay unsupported since they have no static width to bitcast against.

Start with Context::materializeConversion in ImportVerilog and inspect the existing moore.conversion MooreToCore lowering for its width-checked bitcast behavior. Reproduce the provided streaming-array example, then verify that fixed-size aggregates with equal statically known bit sizes convert while queues, dynamic arrays, and associative arrays remain unsupported.

Written by the indexing model from the issue text.