test(e2e): document single-line ASCII constraint on preview sample

e2e/preview-text.test.ts

@@ -6,6 +6,13 @@ import {
 
 test.describe('preview text', () => {
     test('drives the slider character rendering', async ({ comparison }) => {
+        /**
+         * Must stay a single unwrapped line of ASCII: the assertion feeds
+         * `text.length` (UTF-16 code units) to `windowSizeForLine`, but the
+         * renderer feeds it the line's grapheme count. They match only for
+         * plain ASCII — emoji/combining marks (length > graphemes) or wrapping
+         * (one input string splitting into several lines) silently desync them.
+         */
         const text = 'Sphinx';
         await comparison.pickPair('Inter', 'Roboto');
         await comparison.setPreviewText(text);