Improve source positions for lists

```
- a
- b
```

yields
```
doc
list (1:2:2-4:1:11) list_style="-" tight="true"
list_item (1:2:2-2:2:7)
para (1:4:4-2:1:6)
str (1:4:4-1:4:4) text="a"
list_item (2:2:7-4:1:11)
para (2:4:9-2:5:10)
str (2:4:9-2:4:9) text="b"
references = {
}
footnotes = {
}
```
Ideally the first list item would end at 2:1 or 1:4 rather than 2:2.

Reproduce the issue using the two-item list example in the report and inspect the parser's list-handling entry point. Compare the emitted source ranges for the first list item with the expected end positions at 2:1 or 1:4. Done means the source positions are corrected without regressing the surrounding list, paragraph, or string positions.

Written by the indexing model from the issue text.