Bug: Typing within markers slows the app dramatically the more characters exist in a document

Nobody has claimed this yet.

**Describe the bug**
typing any character within dual square brackets freezes the app for a split second, such that typing quickly causes the document to stutter and/or take long enough to update that multiple characters appear simultaneously a few times a second.
this does not improve via restarting the app or closing other tabs. this also does not happen with regular text, so i can type completely fine as long as it's not within dual square brackets.
however, it gets worse the more characters are in a document; i've tested this with documents of various length and it's completely fine in shorter ones - it's only this noticeable in my largest (150,000 character) document. i use a lot of markers in my documents, so this is an issue i run into enough for it to be a little frustrating

**Screenshots**
video:

https://github.com/user-attachments/assets/3cafa026-7af7-4948-a541-0617c3fbbfcb

**macOS version**
Tahoe 26.4
on a 2021 M1 Pro

i love the app, btw! this is easily the best screenwriting application i've found so far. thank you for making it free and open source :D

Reproduce the freeze by typing inside dual square brackets in a roughly 150,000-character document, then compare it with regular text and a shorter document. The payload names no files, tests, or entry points, so trace the marker-related editing path from this reproduction. Done means typing remains responsive as document length and marker count increase.

Written by the indexing model from the issue text.