Safari: fix incorrect cursor position after smart quote insertion

Safari 26 positions the cursor incorrectly after smart quotes text
substitution, causing it to jump back 1-2 characters.

After insertReplacementText events, the cursor is incorrectly
positioned. Trix's selectionDidChange handler syncs, corrupting its
internal state.

Hack to work around it:
* Detect whether replacement is "at cursor" (smart quotes) vs
  "before cursor" (autocorrect) with <= 2 char distance heuristic.
* For at-cursor replacements: manually calculate correct positioning
  accounting for length diff and triggering character (+1); set cursor
  in Trix; skip 3 input events to prevent corrupt sync; prevent
  selectionDidChange from syncing in the meantime; restore cursor
  position in requestAnimationFrame (no visible cursor movement).

To reproduce the issue: enable Edit > Substitutions > Smart Quotes in
Safari, hit Enter twice then "I'll " - including the final space. The
cursor should remain after the final space, no after the replaced
apostrophe. Similarly with autocorrect: the cursor should not move to
the end of the spelling-corrected word.

Author: jeremy