fix: position reference footer below content to prevent overlap (Closes #25)

The reference footer was anchored to a fixed position near the bottom
margin, causing it to overlap with notes text on songs that had both
notes and references (e.g., "Die Gedanken sind frei").

Changed renderReferenceFooter to use flow-based positioning from the
current y-position after notes/metadata, ensuring the separator line,
abbreviation headers, and page numbers always render below the
preceding content without overlap.

Also updated calculateFooterReservation and MeasurementEngine to match
the simplified layout calculation, and added toc.highlight_column to
the example songbook.yaml to demonstrate column highlighting.

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

layout/src/main/kotlin/de/pfadfinder/songbook/layout/MeasurementEngine.kt

@@ -91,11 +91,11 @@ class MeasurementEngine(
             }
         }
 
-        // Reference book footer: reserve space for abbreviation row + page number row + separator line
+        // Reference book footer: gap + separator line + abbreviation row + page number row
         if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty()) {
             val metaLineHeight = fontMetrics.measureLineHeight(config.fonts.metadata, config.fonts.metadata.size)
+            heightMm += 4f * 0.3528f         // gap before footer (4pt converted to mm)
             heightMm += metaLineHeight * 1.4f * 2  // two rows (headers + numbers)
-            heightMm += metaLineHeight * 0.5f  // separator line gap
         }
 
         val pageCount = if (heightMm <= contentHeightMm) 1 else 2