Lower word-split threshold from 7 to 4 chars

Short merged words like "anew" (a new), "Imadea" (I made a),
"makeadecision" (make a decision) were missed because the split
threshold was too high. Now processes tokens >= 4 chars.

English single-letter words (a, I) are already handled by the DP
algorithm which allows them as valid split points.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/grid_editor_api.py

@@ -1751,10 +1751,10 @@ async def _build_grid_core(
                     parts = []
                     changed = False
                     for token in text.split():
-                        # Only try splitting pure-alpha tokens > 7 chars
+                        # Try splitting pure-alpha tokens >= 4 chars
                         clean = token.rstrip(".,!?;:'\")")
                         suffix = token[len(clean):]
-                        if len(clean) > 7 and clean.isalpha():
+                        if len(clean) >= 4 and clean.isalpha():
                             split = _try_split_merged_word(clean)
                             if split:
                                 parts.append(split + suffix)