feat: Broadcasting input to all surfaces in a split/tab/window

[kuroa-me]

What do you want to change?

There is an existing discussion #3227 about broadcasting behavior like iTerm2 (locked for spamming), also #3959 #7086 dups, I would like to implement that.
There is a PR #10531 (closed for missing a linking issue) tried to implement it, but it simply broadcasts to all surfaces in the app, which might not be the desired behavior. Also lacks any indications.

I would like to discuss detailed implementation in a feature request discussion thread, but I will share my current working impl here to increase my chance of passing the vouch.
PoC macOS only here. You will be able to toggle the broadcast input to: window, tab, surface. Use command palette to invoke "Broadcast Input" or set your own keybind keybind = super+shift+i=broadcast_input:current_tab . (No more ctrl+opt+shift+⌘+I for iterm2 toggle, yay)

Details

The core broadcasting will behaves like #10531, but differs in the surface selection phase. Just like iTerm2, the impl will support multiple domains, but limit one domain per window. Encoded text will be broadcasted within the domain.

Since the underlying zig part does only have scope of app and surface. A new unique identifier broadcast_domain is added to surface, and on keyCallback surface will queueIO to all the surfaces that have the same broadcast_domain with the focused surface.

apprt, in my case macOS swift, will be the one who is setting broadcast_domain to the surfaces. Since only runtime will know the actual arrangement of the surfaces. A new optional action and some CAPI are added to support this.

I also added a broadcast budge just like read-only budge, and discovered some funky interactions with read-only mode in the way, but will left this to the feature request discussion if possible.

Why do you want to make this change?

I need the broadcast feature from iTerm2, this is the only thing keeping me from using ghostty, and I would like to switch to ghostty completely.

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.