Rule (a2): switch from allow-list to block-list for symbol removal

Instead of keeping only specific symbols (_KEEP_SYMBOLS), now only
removes explicitly decorative symbols (_REMOVE_SYMBOLS: > < ~ \ ^ etc).
All other punctuation (= ( ) ; : - etc.) is preserved by default.

This is more robust: any new symbol used in textbooks will be kept
unless it's in the small block-list of known decorative artifacts.

Fixes: (= token still being removed on page 5 despite being in
the allow-list (possibly due to Unicode variants or whitespace).

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

klausur-service/backend/grid_build_core.py

@@ -1407,13 +1407,14 @@ async def _build_grid_core(
             # Rule (a2): isolated non-alphanumeric symbols (graphic OCR artifacts)
             # Small images/icons next to words get OCR'd as ">", "<", "~", etc.
             # Remove word boxes that contain NO letters or digits.
-            # Exception: meaningful punctuation used in textbooks (=, ;, :, -, etc.)
+            # Exception: keep standard punctuation and symbols commonly used
-            _KEEP_SYMBOLS = {'=', '(=', '=)', ';', ':', '-', '–', '—', '/', '+',
+            # in textbooks (=, ;, :, -, (, ), etc.). Only remove truly
-                             '•', '·', '(', ')', '&', '*', '→', '←', '↔'}
+            # decorative symbols like >, <, ~, \, ^, `, #.
+            _REMOVE_SYMBOLS = {'>', '<', '~', '\\', '^', '`', '#', '|', '¬', '¦'}
             for i, wb in enumerate(wbs):
                 t = (wb.get("text") or "").strip()
                 if t and not re.search(r'[a-zA-Z0-9äöüÄÖÜß]', t) and len(t) <= 2:
-                    if t not in _KEEP_SYMBOLS:
+                    if t in _REMOVE_SYMBOLS:
                         to_remove.add(i)
 
             # Rule (b) + (c): overlap and duplicate detection