Skip character when moving to start of paragraph with selectionBehavior character

Fixes moving to the previous paragraph with selectionBehavior character, as described in #408. Too see what this does, create a text file with a few parargaphs like ``` foo bar baz ``` and use goto previous paragraph. Before: <img width="134" height="135" alt="image" src="https://github.com/user-attachments/assets/e1a71144-a437-4f6f-a49e-5cab8a17a381" /> After: <img width="136" height="137" alt="image" src="https://github.com/user-attachments/assets/2c2fd9f9-01d7-47f9-9589-2c1f74910b4b" /> This partially resolves #408. There's a similar issue when moving to the next paragraph, although it's far less severe in that case, as it only results in selecting extra whitespace. (Honestly, the precise behavior for moving paragraphs in helix is pretty difficult to understand.)