`Module.uniqueInstanceName` not enforced as reserved name in `Synth` stack

### Describe the bug

The `Module.uniqueInstanceName` is supposed to be a guaranteed unique name for an instance in the context of a module hierarchy. This is nice for debug and waveform viewing, but doesn't come with guarantees on generated outputs.

We should fix the `Synth` stack so that when outputs are generated, the instance names of modules are guaranteed to be equal to the `uniqueInstanceName`. This does mean that if someone tries to reserve another name that would conflict with an auto-selected unique instance name, it will fail on the other reserved name. Since this happens during build, perhaps we can first check for reserved internal signal names to avoid such conflicts.

### To Reproduce

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### Expected behavior

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### Actual behavior

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### Additional: Dart SDK info

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### Additional: pubspec.yaml

```yaml

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### Additional: Context

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