"factor" produces output hard for a program to use

... because the constant factor may be absent, and it always has an
exponent of 1:
```
i20 : peek'_3 factor (1 * (x-1) * (x-2)^2 )

o20 = Product{Power{QQ[x]{x-2}, 2}, Power{QQ[x]{x-1}, 1}}

i21 : peek'_3 factor (2 * (x-1) * (x-2)^2 )

o21 = Product{Power{QQ[x]{x-2}, 2}, Power{QQ[x]{x-1}, 1}, Power{QQ[x]{2}, 1}}
```

No source files or tests are named. Start with the factor and peek'_3 behavior shown in the examples, determine a stable machine-readable representation for factors with absent constants and exponent 1, then add regression coverage demonstrating the intended output.

Written by the indexing model from the issue text.