Can't access proc state if it is a struct

Let's say i want to have a state in a proc that is more complicated. So I'd naturally put it in a
struct:

Let's assume the following change to [`xls/examples/passthrough.x`](https://github.com/google/xls/blob/main/xls/examples/passthrough.x#L38)
in which we replace the count with a struct that contains a count:

```diff
--- a/passthrough.x 2026-09-04 20:32:13.724012164 +0200
+++ b/passthroug.x 2026-09-04 20:34:48.517393354 +0200
@@ -35,23 +35,28 @@
}
}

+struct MyCount {
+ count: u32,
+}
+
#[test]
proc PassthroughTest {
terminator: chan out,
data_s: chan out,
data_r: chan in,
- count: u32,
+ my_state: MyCount,
}

impl PassthroughTest {
fn new(terminator: chan out) -> Self {
let (data_s, data_r) = chan("data");
Passthrough::new(data_r, data_s).spawn();
- PassthroughTest { terminator, data_s, data_r, count: u32:10 }
+ PassthroughTest { terminator, data_s, data_r,
+ my_state: MyCount { count: u32:10} }
}

fn next(self) {
- let count = read(self.count);
+ let count = read(self.my_state.count);
let tok: token = join();
let data_to_send = count * count;
let tok = send(tok, self.data_s, data_to_send);
@@ -59,6 +64,6 @@

assert_eq(data_to_send, received_data);
send_if(tok, self.terminator, count == u32:0, true);
- write(self.count, count - u32:1);
+ write(self.my_state.count, count - u32:1);
}
}
```

This complains with an error message:

```
$ bazel-bin/xls/dslx/interpreter_main passthrough.x
passthrough.x:59:40-59:45
0057:
0058: fn next(self) {
0059: let count = read(self.my_state.count);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^---^ TypeInferenceError: No member `count` in struct `BuiltinProcState`.
0060: let tok: token = join();
0061: let data_to_send = count * count;
```

There is no `BuiltinProcState` anywhere in this file. The struct type in question would be `MyCount`.

This is either something somewhat hardcoded to not be able to treat the fields as what they are, not ready yet, or I am holding it wrong :). If so, how do I hold it right ?

Reproduce the issue with the modified xls/examples/passthrough.x using bazel-bin/xls/dslx/interpreter_main. Start by tracing the type-inference path for self.my_state.count and the reported BuiltinProcState error. Done means the MyCount field is recognized correctly and the example runs without that TypeInferenceError.

Written by the indexing model from the issue text.