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

renderer-pdf/src/main/kotlin/de/pfadfinder/songbook/renderer/pdf/PdfBookRenderer.kt

@@ -240,12 +240,11 @@ class PdfBookRenderer : BookRenderer {
             }
         }
 
-        // Reference footer
+        // Reference footer: gap + separator line + abbreviation row + page number row
         if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty()) {
             val lineHeight = metaSize * 1.4f
+            reserved += 4f              // gap before footer
             reserved += lineHeight * 2  // two rows (headers + numbers)
-            reserved += lineHeight * 1.5f  // separator line gap
-            reserved += lineHeight * 0.5f  // bottom gap
         }
 
         return reserved
@@ -474,10 +473,10 @@ class PdfBookRenderer : BookRenderer {
 
         // Render reference book footer on the last page of the song
         if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty() && isLastPage) {
+            y -= 4f  // gap before reference footer
             renderReferenceFooter(
                 cb, fontMetrics, config, song,
-                leftMargin, contentWidth,
-                bottomMargin
+                leftMargin, y, contentWidth
             )
         }
     }
@@ -512,8 +511,9 @@ class PdfBookRenderer : BookRenderer {
     }
 
     /**
-     * Renders reference book abbreviations and page numbers as a footer row
-     * at the bottom of the song page, above the page number.
+     * Renders reference book abbreviations and page numbers as a footer
+     * on the song page, positioned below the current content at [topY].
+     * Layout (top to bottom): separator line, abbreviation row, page number row.
      */
     private fun renderReferenceFooter(
         cb: PdfContentByte,
@@ -521,40 +521,44 @@ class PdfBookRenderer : BookRenderer {
         config: BookConfig,
         song: Song,
         leftMargin: Float,
-        contentWidth: Float,
-        bottomMargin: Float
+        topY: Float,
+        contentWidth: Float
     ) {
         val metaFont = fontMetrics.getBaseFont(config.fonts.metadata)
         val metaSize = config.fonts.metadata.size
         val lineHeight = metaSize * 1.4f
 
-        // Position: just above the page number area
-        val footerY = bottomMargin + lineHeight * 0.5f
-
-        // Map book IDs to abbreviations
-        val refAbbreviations = config.referenceBooks.associate { it.id to it.abbreviation }
         val books = config.referenceBooks
 
         // Calculate column widths: evenly distribute across content width
         val colWidth = contentWidth / books.size
 
-        // Row 1: Abbreviation headers
+        // Draw a thin separator line at the top of the footer
+        val lineY = topY
+        cb.setLineWidth(0.3f)
+        cb.setColorStroke(Color.LIGHT_GRAY)
+        cb.moveTo(leftMargin, lineY)
+        cb.lineTo(leftMargin + contentWidth, lineY)
+        cb.stroke()
+
+        // Row 1: Abbreviation headers (below the separator line)
+        val abbrY = lineY - lineHeight
         for ((i, book) in books.withIndex()) {
             val x = leftMargin + i * colWidth
             val abbr = book.abbreviation
             val textWidth = metaFont.getWidthPoint(abbr, metaSize)
-            // Center text in column
             val textX = x + (colWidth - textWidth) / 2
 
             cb.beginText()
             cb.setFontAndSize(metaFont, metaSize)
             cb.setColorFill(Color.DARK_GRAY)
-            cb.setTextMatrix(textX, footerY + lineHeight)
+            cb.setTextMatrix(textX, abbrY)
             cb.showText(abbr)
             cb.endText()
         }
 
-        // Row 2: Page numbers
+        // Row 2: Page numbers (below abbreviation headers)
+        val numY = abbrY - lineHeight
         for ((i, book) in books.withIndex()) {
             val x = leftMargin + i * colWidth
             val pageNum = song.references[book.id]
@@ -566,18 +570,11 @@ class PdfBookRenderer : BookRenderer {
                 cb.beginText()
                 cb.setFontAndSize(metaFont, metaSize)
                 cb.setColorFill(Color.DARK_GRAY)
-                cb.setTextMatrix(textX, footerY)
+                cb.setTextMatrix(textX, numY)
                 cb.showText(pageText)
                 cb.endText()
             }
         }
-
-        // Draw a thin line above the footer
-        cb.setLineWidth(0.3f)
-        cb.setColorStroke(Color.LIGHT_GRAY)
-        cb.moveTo(leftMargin, footerY + lineHeight * 1.5f)
-        cb.lineTo(leftMargin + contentWidth, footerY + lineHeight * 1.5f)
-        cb.stroke()
     }
 
     private fun renderForewordPage(

songbook.yaml

@@ -37,6 +37,9 @@ reference_books:
     name: "Pfadfinderliederbuch"
     abbreviation: "PfLB"
 
+toc:
+  highlight_column: "Seite"
+
 output:
   directory: "./output"
   filename: "liederbuch.pdf"