EU marked by spellchecker despite being in dictionary

**Describe the bug**
The word "EU" is marked as misspelled despite appearing in Harper's dictionary.

**To Reproduce**
Steps to reproduce the behavior:
1. Type "EU" in a text box that supports Harper.
2. See error

**Expected behavior**
EU stands for European Union and is correctly spelled (analogous to US, USA, etc).

**Screenshots**
Not applicable.

**Platform**
Harper core, Obsidian, etc.

**Additional context**
None.

Start by reproducing the issue in Harper core with the input "EU" and trace how dictionary entries are checked. Done means the spellchecker accepts "EU" without marking it as misspelled, while preserving the expected behavior for comparable abbreviations such as "US" and "USA".

Written by the indexing model from the issue text.