fix: prevent colored text from being falsely detected as graphics

Add color pixel density checks to cv_graphic_detect.py Pass 1:
- density < 20% → skip (text strokes are thin, images are filled)
- density < 30% + height < 4% page → skip (colored text line)

This fixes green headings (Insel, Internet, Inuit) being removed
as graphic regions, which also caused word reordering in lines.

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

klausur-service/backend/cv_graphic_detect.py

@@ -195,6 +195,30 @@ def detect_graphic_elements(
         if color_pixel_count < 200:
             continue
 
+        # Color pixel density: fraction of bbox filled with colored pixels.
+        # Text strokes are thin → low density (typically 5-20%).
+        # Actual images/graphics are filled → high density (30%+).
+        density = color_pixel_count / bbox_area if bbox_area > 0 else 0
+
+        # Very low density → almost certainly colored text, not an image
+        if density < 0.20:
+            logger.debug(
+                "GraphicDetect PASS1 skip low-density (%d,%d) %dx%d "
+                "density=%.0f%% (likely colored text)",
+                bx, by, bw, bh, density * 100,
+            )
+            continue
+
+        # Moderate density + small height → likely a colored text line
+        # (text-line height is typically < 3% of page height)
+        if density < 0.30 and bh < h * 0.04:
+            logger.debug(
+                "GraphicDetect PASS1 skip text-height region (%d,%d) %dx%d "
+                "density=%.0f%% height=%.1f%% (likely colored text line)",
+                bx, by, bw, bh, density * 100, 100.0 * bh / h,
+            )
+            continue
+
         # Determine dominant color from the actual colored pixels
         roi_hsv = hsv[by:by + bh, bx:bx + bw]
         color_px_mask = roi_color > 0
@@ -205,11 +229,10 @@ def detect_graphic_elements(
             color_name, color_hex = "black", _COLOR_HEX["black"]
 
         # Confidence based on color density and low word overlap
-        density = color_pixel_count / bbox_area if bbox_area > 0 else 0
         conf = min(0.95, 0.5 + density * 0.5)
 
-        logger.debug("GraphicDetect PASS1 accept (%d,%d) %dx%d px=%d overlap=%.0f%% %s",
+        logger.debug("GraphicDetect PASS1 accept (%d,%d) %dx%d px=%d density=%.0f%% overlap=%.0f%% %s",
-                     bx, by, bw, bh, color_pixel_count, word_overlap * 100, color_name)
+                     bx, by, bw, bh, color_pixel_count, density * 100, word_overlap * 100, color_name)
         candidates.append(GraphicElement(
             x=bx, y=by, width=bw, height=bh,
             area=color_pixel_count,