Tighten tertiary column detection: require 4+ rows and 5% coverage

Prevents false narrow columns from text overflow at page edges.
Session 355f3c84 had a 3-row/4% tertiary cluster creating a spurious
third column from right-column text overflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

klausur-service/backend/grid_editor_api.py

@@ -183,9 +183,15 @@ def _cluster_columns_by_alignment(
     used_ids = {id(c) for c in primary} | {id(c) for c in secondary}
     sig_xs = [c["mean_x"] for c in primary + secondary]
 
+    MIN_DISTINCT_ROWS_TERTIARY = max(MIN_DISTINCT_ROWS + 1, 4)
+    MIN_COVERAGE_TERTIARY = 0.05  # at least 5% of rows
     tertiary = []
     for c in clusters:
-        if id(c) in used_ids or c["distinct_rows"] < MIN_DISTINCT_ROWS:
+        if id(c) in used_ids:
+            continue
+        if c["distinct_rows"] < MIN_DISTINCT_ROWS_TERTIARY:
+            continue
+        if c["row_coverage"] < MIN_COVERAGE_TERTIARY:
             continue
         # Must be near left or right content margin (within 15%)
         rel_pos = (c["mean_x"] - content_x_min) / content_span if content_span else 0.5