Wrong stereo-to-mono copy/paste behavior

### Steps to reproduce

1. Generate chirp
2. Add a stereo track and generate chirp
3. Select and copy a region of the stereo chirp
4. Select a region inside the mono chirp
5. Ctrl (Cmd) + V

### Expected behavior

### Actual behavior

An attempt to paste the copied data into the mono track pastes it into the stereo track.

https://github.com/user-attachments/assets/b3568d84-0c18-4051-b20f-bee8210a5a0c

### Audacity Version

Audacity 3.7.x

### Operating system

Windows, macOS, Linux

### Additional context

This can only be reproduced if the mono track comes first, and the stereo track second.

Reproduce the issue in Audacity 3.7.x using the listed chirp, mono-track, stereo-track, and Ctrl/Cmd+V steps on a supported desktop platform. Start at the paste action and track-selection handling, then verify that copied stereo data is pasted into the selected mono region when the mono track comes first; done means the reported behavior no longer occurs across Windows, macOS, and Linux.

Written by the indexing model from the issue text.