Crouch/Prone visibility when adjacent to cover

### Describe the bug

While reading #54635, I decided to test something that was talked about when prone was first introduced: how crouching while driving make you basically invisible.

And it still works, as long as you're adjacent to something you can't walk through. Line of sight blocked, great!

But if you all of a sudden go prone, you lose that line of sight concealment. Which is weird.

### Steps to reproduce

Have a vehicle that has a 3x3 aisle in the middle, and is surrounded by quarterpanels and windshield. A door too, if you need it.

Stand beside them, and look through.
Crouch, and be lose line of side through what you're directly adjacent to.
Go Prone, and look out the crack at the bottom of the wall, I guess?

### Expected behavior

Prone should provide at least the same line of sight disruptions that crouching does.

### Screenshots


### Versions and configuration

Win 10, tiles, e49949d

### Additional context

_No response_

Start by reproducing issue #54643 with the described 3x3 vehicle aisle, quarterpanels, and windshield, comparing crouching with prone line of sight. Trace the existing visibility handling for those two stances and verify that prone provides at least the same concealment as crouching in the adjacent-cover case.

Written by the indexing model from the issue text.