Fix combineSpans

```
> combineSpans [Span (Range (Coord 0 0) (Coord 0 2)) "a", Span (Range (Coord 0 1) (Coord 0 1)) "b"]
[((Coord (row 0) (col 0)),"a"),((Coord (row 0) (col 1)),"ab"),((Coord (row 0) (col 1)),"a"),((Coord (row 0) (col 2)),"")]
```

Representing this in ASCII

```
“aaa”
“ b ”
—— currently combines to
b
a
“aa.“
```
Where vertical stacking is `mappend` and `.` is `mempty`

Where we would expect:
```
“aaa”
“ b ”
—— combines to
b
“aaa“
```

Search the repository for the combineSpans definition and inspect how the example spans are combined. Compare the current ASCII result with the expected result in this issue, including the overlapping and empty positions. Done means combineSpans produces the expected stacked output for this example.

Written by the indexing model from the issue text.