RingMap MutableMatrix is broken

Here's an experiment on 1.21:
```m2
i1 : C = toField(QQ[i]/(i^2+1))

o1 = C

o1 : PolynomialRing

i2 : R = C[x]

o2 = R

o2 : PolynomialRing
```
So far so good. The first curious thing is the presentation of this map. What is `{-1, i}` doing here?
```m2
i3 : f = map(R, R, {i^2})

o3 = map (R, R, {-1, i})

o3 : RingMap R <--- R

i4 : f x

o4 = -1

o4 : R

i5 : f matrix{{x}}

o5 = | -1 |

1 1
o5 : Matrix R <--- R
```
And now we get to the bug:
```m2
i6 : f mutableMatrix{{x}}

o6 = 0

o6 : MutableMatrix
```
`flattenRing R` returns itself, so there's not much to do there.

Reproduce the Macaulay2 session in the issue, focusing on the difference between applying the map to matrix{{x}} and mutableMatrix{{x}}. Trace the MutableMatrix mapping path and compare it with the ordinary matrix result; done means f mutableMatrix{{x}} preserves the mapped entry instead of returning 0, while the existing scalar and matrix behavior remains correct.

Written by the indexing model from the issue text.