False Positive: 'It's' -> 'Its' (`ItsPossessive`)

**What got flagged?**
`It's`

**Why is this incorrect?**
Here, `It's` _is_ being used as a contraction of 'it' and 'is'

**Example of correct usage:**
```md
It's tonight.
```

Locate the Rust implementation of the ItsPossessive rule and reproduce the reported Markdown example, `It's tonight.` Confirm why the contraction is flagged, then verify that the example is accepted without regressing possessive-usage checks.

Written by the indexing model from the issue text.