fuzzer generate non-combinational code w/ --generator=combinational options

XLS fuzzer crasher: codegen_main UNIMPLEMENTED error.

### Error Message
```
Error: UNIMPLEMENTED: Proc combinational generator only supports streaming output channels which can be determined to be mutually exclusive, got 2 output channels which were not proven to be mutually exclusive; Running pass #6: Channel I/O to port lowering pass [short: channel_to_port_io_lowering]; Failed as part of compound pass block_conversion #0; Running pass #0: Top level codegen v1.5 block conversion pipeline [short: block_conversion]
```

### Minimized DSLX
```dslx
proc main {
x6: chan out;
x18: chan out;
config(x6: chan out, x18: chan out) {
(x6, x18)
}
init {
()
}
next(x0: ()) {
let tok: token = join();
let x7: token = send_if(tok, x6, true, true);
let x19: token = send_if(tok, x18, true, u5:1);
x0
}
}
```

### Reproduction Steps
Run `run_crasher` with the minimized DSLX and the following options (which match the crasher config):
`--generator=combinational`

### Root Cause Analysis
The failure occurs in `xls/codegen_v_1_5/channel_to_port_io_lowering_pass.cc` at line 1668.

```cpp
if (needs_one_shot_logic) {
if (options.codegen_options.generate_combinational()) {
return absl::UnimplementedError(absl::StrFormat(
"Proc combinational generator only supports streaming output "
"channels which can be determined to be mutually exclusive, got %d "
"output channels which were not proven to be mutually exclusive",
outgoing_channel_count));
}
```

If a proc has multiple streaming output channels (`outgoing_channel_count > 1`) and they cannot be proven to be mutually exclusive (`AreStreamingOutputsMutuallyExclusive` returns false), the compiler needs to insert "one-shot logic" to ensure each output is sent at most once.

However, one-shot logic requires state (registers) to track which sends have completed.
In combinational mode (`options.codegen_options.generate_combinational()` is true), we cannot insert registers.
Therefore, the combinational generator cannot support this case and returns `UnimplementedError`.