Fuzzer crash 2026-09-14_ce8a

Repro:

```
bazel test -c opt //xls/fuzzer:run_crasher_test_2026-09-14_ce8a
```

Error message:

```
Error: NOT_FOUND: Output `out`, instance #127 holds X value in Verilog simulator output.
=== Source Location Trace: ===
xls/simulation/module_testbench.cc:370
xls/simulation/module_testbench.cc:456
xls/simulation/module_simulator.cc:510
xls/simulation/module_simulator.cc:550
xls/tools/simulate_module_main.cc:208

```

Start by running bazel test -c opt //xls/fuzzer:run_crasher_test_2026-09-14_ce8a and inspect the reported trace, beginning with xls/simulation/module_testbench.cc and xls/simulation/module_simulator.cc. Determine why the Verilog simulator output contains an X for instance #127, then verify that the crasher test completes without the NOT_FOUND error.

Written by the indexing model from the issue text.