Skip setting overlays on collapsed fragments

[AdamNiederer]

Partial resolution of #584

Before:

1,868,011,162  93% + timer-event-handler

         8541  48% + timer-event-handler
         6263  35%   Automatic GC
         2373  13% + redisplay_internal (C function)

After:

 979,703,965  88% + timer-event-handler

        7261  39% + redisplay_internal (C function)
        5444  29% + timer-event-handler
        5411  29%   Automatic GC

redisplay is no longer throttled and we save ~1GB of garbage on a 32k fragment length. CPU usage is still high but it's no longer causing major UI stutter. Expanded fragments still need help and there's probably some more optimization that could be done for both cases.

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• N/A I've filed a feature request/discussion for a new feature.
• N/A I'm making visual changes, so I'm including screenshots so you can view and discuss.
• N/A I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[xenodium]

agent-shell-ui.el is fairly generic in enabling buffer updates through collapsable fragements. With this change, we'd be introducing client-specific logic.

I'd prefer if we found a way to keep markdown-specific logic in agent-shell.el. Maybe through an agent-shell-ui-post-toggle-fragment-at-point-function (hook) that we funcall from here? Happy to consider alternatives.

[xenodium]

Thanks a lot for the PR! Source block highlighting in markdown-overlays.el is highly inneficient. I'm working on a separate markdown rendering solution but that will take a little more time. I'll push it to a branch somewhat soon.

WRT to the PR itself, I'm happy to merge for the time being but could we please address the comment in review? Alternatively, isn't agent-shell-highlight-blocks set to nil enough to improve the immediate issue?

[xenodium]

Thank you for the follow-up changes. Could you resolve the conflicts so they can merged?