feat: display reference book page numbers in song page footer (Closes #3)

Render a two-row footer at the bottom of each song page showing reference
book abbreviations as column headers with corresponding page numbers below.
A thin separator line is drawn above the footer. MeasurementEngine now
reserves vertical space for the reference footer when reference books are
configured and the song has references.

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

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

@@ -241,6 +241,87 @@ class PdfBookRenderer : BookRenderer {
                 y -= metaSize * 1.5f
             }
         }
+
+        // Render reference book footer on the last page of the song
+        if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty()) {
+            val isLastPage = (pageIndex == 0) // For now, all content renders on page 0
+            if (isLastPage) {
+                renderReferenceFooter(
+                    cb, fontMetrics, config, song,
+                    leftMargin, contentWidth,
+                    config.layout.margins.bottom / 0.3528f
+                )
+            }
+        }
+    }
+
+    /**
+     * Renders reference book abbreviations and page numbers as a footer row
+     * at the bottom of the song page, above the page number.
+     */
+    private fun renderReferenceFooter(
+        cb: PdfContentByte,
+        fontMetrics: PdfFontMetrics,
+        config: BookConfig,
+        song: Song,
+        leftMargin: Float,
+        contentWidth: Float,
+        bottomMargin: 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
+        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.showText(abbr)
+            cb.endText()
+        }
+
+        // Row 2: Page numbers
+        for ((i, book) in books.withIndex()) {
+            val x = leftMargin + i * colWidth
+            val pageNum = song.references[book.id]
+            if (pageNum != null) {
+                val pageText = pageNum.toString()
+                val textWidth = metaFont.getWidthPoint(pageText, metaSize)
+                val textX = x + (colWidth - textWidth) / 2
+
+                cb.beginText()
+                cb.setFontAndSize(metaFont, metaSize)
+                cb.setColorFill(Color.DARK_GRAY)
+                cb.setTextMatrix(textX, footerY)
+                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(