fix: use nearest edges for gap calculation, add positive-control test

Address CodeRabbit review feedback:
- Gap calculation now uses nearest edges (bottomY-topY) instead of outer
  edges, preventing multi-line footers with <30pt actual spacing from
  being incorrectly rejected.
- Add testCloseFooterLinesAreGrouped to verify that two footer lines
  11pt apart are grouped into a single footer element.

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

Author: bundolee

java/opendataloader-pdf-core/src/main/java/org/opendataloader/pdf/processors/HeaderFooterProcessor.java

@@ -203,9 +203,9 @@ private static boolean isAdjacentToExistingHeaderOrFooter(
         IObject previousElement = pageContents.get(previousIndex);
         double gap;
         if (isHeaderDetection) {
-            gap = previousElement.getTopY() - candidate.getBottomY();
+            gap = previousElement.getBottomY() - candidate.getTopY();
         } else {
-            gap = candidate.getTopY() - previousElement.getBottomY();
+            gap = candidate.getBottomY() - previousElement.getTopY();
         }
         return gap <= MAX_HEADER_FOOTER_GAP;
     }

java/opendataloader-pdf-core/src/test/java/org/opendataloader/pdf/processors/HeaderFooterProcessorTest.java

@@ -156,4 +156,46 @@ public void testRepeatedBodyTextNotAbsorbedIntoFooter() {
                 "Page " + page + ": repeated note text should remain in body, not be absorbed into footer");
         }
     }
+
+    /**
+     * Positive control: two closely spaced footer lines (gap < 30pt) should be
+     * grouped into a single footer. Ensures the proximity check does not reject
+     * legitimate multi-line footers.
+     */
+    @Test
+    public void testCloseFooterLinesAreGrouped() {
+        initContainers();
+        List<List<IObject>> contents = new ArrayList<>();
+        for (int page = 0; page < 3; page++) {
+            List<IObject> pageContents = new ArrayList<>();
+            // Body text at top
+            pageContents.add(new TextLine(new TextChunk(
+                new BoundingBox(page, 37.0, 500.0, 300.0, 530.0),
+                "Body text page " + (page + 1), 10, 530.0)));
+
+            // Two footer lines close together (11pt gap between nearest edges)
+            // Line 1: y=[55, 67]  Line 2: y=[35, 44]  gap = 55-44 = 11pt
+            pageContents.add(new TextLine(new TextChunk(
+                new BoundingBox(page, 37.0, 55.0, 280.0, 67.0),
+                "Copyright 2026", 7.5, 67.0)));
+            pageContents.add(new TextLine(new TextChunk(
+                new BoundingBox(page, 37.0, 35.0, 280.0, 44.0),
+                "Company Footer", 7.5, 44.0)));
+
+            contents.add(pageContents);
+        }
+
+        HeaderFooterProcessor.processHeadersAndFooters(contents, false);
+
+        for (int page = 0; page < 3; page++) {
+            List<IObject> pageContent = contents.get(page);
+            IObject lastElement = pageContent.get(pageContent.size() - 1);
+            Assertions.assertTrue(lastElement instanceof SemanticHeaderOrFooter,
+                "Page " + page + ": last element should be footer");
+            SemanticHeaderOrFooter footer = (SemanticHeaderOrFooter) lastElement;
+            Assertions.assertEquals(SemanticType.FOOTER, footer.getSemanticType());
+            Assertions.assertEquals(2, footer.getContents().size(),
+                "Page " + page + ": footer should contain both close footer lines (gap=11pt < 30pt)");
+        }
+    }
 }