## Direction predicate

Every `fabric.boundary` op carries a mandatory `direction` attribute
printed/parsed as a bracketed predicate, mirroring `fabric.pe`'s
`[spatial|temporal]` style:

```mlir
fabric.boundary [s2t] ...
fabric.boundary [t2t] ...
fabric.boundary [t2s] ...
```

The keyword between `[` and `]` is one of `s2t`, `t2t`, `t2s`. The
parser rejects any other keyword.