Cache output of flattenRing

```m2
i1 : A = kk[a]; B = A[b];

i3 : X = first flattenRing B;

i4 : Y = flattenRing(B, Result => 1);

i5 : X === Y

o5 = false
```

From documentation:
> Multiple calls to flattenRing may result in defining multiple rings that are regarded as different objects by Macaulay2.

This seems to be easily avoidable.

Start by reproducing the Macaulay2 transcript in the issue and locate the flattenRing entry point. Trace the repeated calls and their Result => 1 behavior, then verify that equivalent calls reuse the same ring object so the final X === Y check succeeds.

Written by the indexing model from the issue text.