Map of multigraded modules removes degrees

Nobody has claimed this yet.

Here is an example:
```m2
A = kk[x_0..x_1, DegreeRank => 2]
B = kk[a_0..a_1,y_0..y_1, DegreeRank => 4]
f = map(B, A, {B_0, B_1})
degrees A^{{1,2}} -- {{-1, -2}}
degrees B^{{1,2,0,0}} -- {{-1, -2, 0, 0}}
degrees f A^{{1,2}} -- {{ 0, 0, 0, 0}}, seems wrong
degrees sub(A^{{1,2}}, B) -- same
```

Start by reproducing the exact Macaulay2 example in the issue and inspect the behavior of degrees on the map and submodule expressions. Trace the implementation responsible for multigraded module degree conversion; done means the mapped and submodule degree outputs preserve the expected four-component grading rather than becoming all zeros.

Written by the indexing model from the issue text.