Root cause (per Opus analysis): UAZoomEnabled() is a synchronous
Mach IPC roundtrip to macOS Accessibility server, called 3x per
redisplay cycle. At 60fps = 180 IPC roundtrips/second blocking the
main thread. Combined with Emacs's inherent O(position) redisplay
cost, this compounded into progressive choppy behavior.
Fix 1: ns_zoom_enabled_p() caches UAZoomEnabled() for 1 second.
Fix 2: ns_zoom_track_completion() rate-limited to 2 Hz.
Also includes BUF_CHARS_MODIFF fix (patch 0009) for VoiceOver cache.
BUF_CHARS_MODIFF fix — the core performance regression:
ensureTextCache checked BUF_MODIFF which font-lock bumps on every
redisplay. Each cursor movement in a large file triggered full buffer
rebuild. Now uses BUF_CHARS_MODIFF (changes only on char insert/delete).
