terminal: report selected match bounds

[bo2themax]

I wanted to implement findIndicator-like highlights when searching, in order to do this I need the coordinates for the selected matches, in order to apply the animation in the app.

This pr is the first step which includes the core terminal changes to report those bounds from render.

The bounds info extends current ghostty_action_search_selected_s
struct, so gtk can implement it whenever it's appropriate.

I also tried to make this as a separate event like search_selected_regions, which would still achieve the goal, but then I thought it's better to keep message together with search_selected since these bounds only make sense when the index is not null.
The result (current) is a bit more changes, but it makes more sense to me.

The second commit adds a change reason to ghostty_action_search_selected_s, which will help apprt to distinguish changes caused only by navigate_search.

Screen.Recording.2026-06-07.at.13.58.33.mp4

AI Disclosure

Claude implemented this, I read all the changes myself, and tested it as much as I can with normal search. Also different Claude sessions reviewed and refined multiple rounds based on my suggestions.