fix(search): cap an expression at 256 atoms

The compiled SQL chains every atom into one expression, and SQLite rejects
expressions deeper than 1000 levels. A query of about 1000 words parsed but
then failed in SQL, outside the syntax-error handling, and crashed the app.
The parser now rejects more than 256 atoms as a syntax error.

USAGE.md

@@ -226,6 +226,7 @@ These are invalid:
 |---|---|
 | Unbalanced parentheses | `(tag:bug`, `tag:bug)` |
 | Parentheses nested more than 64 levels deep | `((((…a…))))` with 65 `(` |
+| More than 256 search terms (words, phrases and qualifiers) | 257 words |
 | An operator with a missing side | `bug AND`, `OR bug` |
 | Consecutive operators | `a AND OR b` |
 | A qualifier without a value | `tag:` |