2D array channel out shows errors like it was not constexpr: `c_out`: INVALID_ARGUMENT

**Describe the bug**
When adapt the example DSLX code `matmul_4x4.x`. I add another 2D array channel out (c_out: chan[3][3] out). But it gives me the error like `not constexpr: 'c_out': INVALID_ARGUMENT`. I didn't know where it needs this constexpr attribute.

**To Reproduce**
Steps to reproduce the behavior:
```
...
// well defined `proc node` here.
...

proc matmul {
// Public Interface:
a_in: chan[ROWS] in;
b_in: chan[COLS] in;
c_out: chan[COLS][ROWS] out;
screenshot_reset: chan in;

// Private (Internal) Members:
west_inputs: chan[COLS + u32:1][ROWS] in;
north_inputs: chan[COLS][ROWS + u32:1] in;
east_outputs: chan[COLS + u32:1][ROWS] out;
south_outputs: chan[COLS][ROWS + u32:1] out;

config(a_in: chan[ROWS] in, b_in: chan[COLS] in,
c_out: chan[COLS][ROWS] out, screenshot_reset: chan in) {
// Declare internal east-to-west channels and south-to-north channels.
let (east_outputs, west_inputs) = chan[COLS + u32:1][ROWS]("east_west");
let (south_outputs, north_inputs) = chan[COLS][ROWS + u32:1]("south_north");

unroll_for! (row, _): (u32, ()) in u32:0..ROWS {
unroll_for! (col, _): (u32, ()) in u32:0..COLS {
spawn node(
west_inputs[row][col], north_inputs[row][col], east_outputs[row][col + u32:1],
south_outputs[row + u32:1][col], screenshot_reset,
c_out[row][col]);
}(());
}(());

(a_in, b_in, c_out, screenshot_reset, west_inputs, north_inputs, east_outputs, south_outputs)
}

init { () }

next(state: ()) {
// Send input matricies:
let zero = u32:0;
unroll_for! (row, _): (u32, ()) in u32:0..ROWS {
let (tok, local_a) = recv(join(), a_in[row]);
send(tok, east_outputs[row][zero], local_a);
}(());
unroll_for! (col, _): (u32, ()) in u32:0..COLS {
let (tok, local_b) = recv(join(), b_in[col]);
send(tok, south_outputs[zero][col], local_b);
}(());

// No useful outputs from the other side (Drop):
// TODO - google/xls#1750: remove unroll_for! workaround.
unroll_for! (cols, _): (u32, ()) in COLS..COLS + u32:1 {
unroll_for! (row, _): (u32, ()) in u32:0..ROWS {
recv(join(), west_inputs[row][cols]);
}(());
}(());
unroll_for! (rows, _): (u32, ()) in ROWS..ROWS + u32:1 {
unroll_for! (col, _): (u32, ()) in u32:0..COLS {
recv(join(), north_inputs[rows][col]);
}(());
}(());
}
}

// proc matmul_3x3 {
// config(a_in: chan[3] in, b_in: chan[3] in,
// c_out: chan[3][3] out, screenshot_reset: chan in) {
// spawn matmul(a_in, b_in, c_out, screenshot_reset);
// }

// init { () }

// next(state: ()) { }
// }
```

**Expected behavior**

If you remove the comment sign before this part :
```
proc matmul_3x3 {
config(a_in: chan[3] in, b_in: chan[3] in,
c_out: chan[3][3] out, screenshot_reset: chan in) {
spawn matmul(a_in, b_in, c_out, screenshot_reset);
}

init { () }

next(state: ()) { }
}
```
Then when you call it by `spawn matmul_3x3(a_in, b_in, c_out, screenshot_reset);` in the following test_proc, it will give you error like this:
```
$ ./bazel-bin/xls/dslx/interpreter_main E3_F32_matrixmul4_3_2025.x
E0106 16:52:13.244860 19019 command_line_utils.cc:50] Could not extract a textual position from error message: INVALID_ARGUMENT: Expression @ E3_F32_matrixmul4_3_2025.x:126:48-126:53 was not constexpr: `c_out`: INVALID_ARGUMENT: Provided status is not in recognized error form: INVALID_ARGUMENT: Expression @ E3_F32_matrixmul4_3_2025.x:126:48-126:53 was not constexpr: `c_out`
Error: INVALID_ARGUMENT: Expression @ E3_F32_matrixmul4_3_2025.x:126:48-126:53 was not constexpr: `c_out`
```
but if I remove that layer of abstraction by calling the `spawn matmul(a_in, b_in, c_out, screenshot_reset);` myself, it will not give any problems. Why?
**I am confusing where do I use constexpr in my code? Why it doesn't work if I define 2D output channels? But it still work well if I only define 1D output channels like example code from your github repo.**