### Input-mux fields

One typed record per active-FU input describes how a PE input is routed onto
that FU input. The static field inventory contains the corresponding record for
every concrete FU input, while the configured view projects only the selected
FU's records. A value is one member of a closed typed sum:

```text
InputSelection = Disconnected
               | Route(FabricTransportEndpointRef)
               | Discard(FabricTransportEndpointRef)
```

* `Route` selects one of this PE's `K` Input-role transport endpoints and
  connects it to the FU input.
* `Discard` drains the selected PE input locally:
  the FU input's `valid` is forced low and the selected PE input's
  `ready` is forced high so upstream tokens dissipate.
* `Disconnected` makes the FU input inert: no PE input is selected and the FU
  input's `valid` is forced low.