`SpellCheck` shouldn't suggest (only) lowercase replacements for all-caps mistakes

It's very common to write for stylistic reasons in all-caps (or all-lowercase, or all-titlecase) but when `SpellCheck` finds a misspelled word in the middle of an all-caps sentence, the fixes it suggests are all lowercase.

The suggestions should be in the same case as either the mistaken word or the nearby words.

Start by tracing the SpellCheck suggestion-generation entry point and reproduce an all-caps misspelling alongside lowercase and titlecase examples. Look for the existing suggestion tests, which the issue does not name, and verify that suggested fixes preserve the casing of the mistaken word or nearby words.

Written by the indexing model from the issue text.