Fix marker column detection: remove min-rows requirement

Words to the left of the first detected column boundary must always
form their own column, regardless of how few rows they appear in.
Previously required 4+ distinct rows for tertiary (margin) columns,
which missed page references like p.62, p.63, p.64 (only 3 rows).

Now any cluster at the left/right margin with a clear gap to the
nearest significant column qualifies as its own column.

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

klausur-service/backend/grid_editor_helpers.py

@@ -375,13 +375,17 @@ 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)
+    # Tertiary: clusters that are clearly to the LEFT of the first
-    MIN_COVERAGE_TERTIARY = 0.05  # at least 5% of rows
+    # significant column (or RIGHT of the last).  If words consistently
+    # start at a position left of the established first column boundary,
+    # they MUST be a separate column — regardless of how few rows they
+    # cover.  The only requirement is a clear spatial gap.
+    MIN_COVERAGE_TERTIARY = 0.02  # at least 1 row effectively
     tertiary = []
     for c in clusters:
         if id(c) in used_ids:
             continue
-        if c["distinct_rows"] < MIN_DISTINCT_ROWS_TERTIARY:
+        if c["distinct_rows"] < 1:
             continue
         if c["row_coverage"] < MIN_COVERAGE_TERTIARY:
             continue