Unknown initialisms/acronyms are marked as spelling errors

**What got flagged?**
Multiple all-capital `RMS` (Root Mean Squared) as well as all-capital `UT` representing a domain specific acronym, are marked with "Did you mean to spell `<>` this way?"

**Why is this incorrect?**
RMS is a common engineering term of art, and user-specific initialisms and acronyms should either be treated as such or have a distinct dictionary to inform harper they are not spelling errors.

**Example of correct usage:**
N/A

Start by reproducing the spelling diagnostics for the all-capital terms `RMS` and `UT` and trace the acronym handling in Harper. The issue does not name files or tests; done requires agreeing on whether unknown initialisms are accepted or supported through a distinct dictionary, then verifying that these examples are no longer flagged incorrectly.

Written by the indexing model from the issue text.