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>

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

@@ -66,6 +66,13 @@ class MeasurementEngine(
             }
         }
 
+        // Reference book footer: reserve space for abbreviation row + page number row + separator line
+        if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty()) {
+            val metaLineHeight = fontMetrics.measureLineHeight(config.fonts.metadata, config.fonts.metadata.size)
+            heightMm += metaLineHeight * 1.4f * 2  // two rows (headers + numbers)
+            heightMm += metaLineHeight * 0.5f  // separator line gap
+        }
+
         val pageCount = if (heightMm <= contentHeightMm) 1 else 2
         return MeasuredSong(song, heightMm, pageCount)
     }

layout/src/test/kotlin/de/pfadfinder/songbook/layout/MeasurementEngineTest.kt

@@ -258,4 +258,70 @@ class MeasurementEngineTest {
 
         labeledHeight shouldBeGreaterThan unlabeledHeight
     }
+
+    @Test
+    fun `references add footer height when reference books configured`() {
+        val configWithRefs = BookConfig(
+            referenceBooks = listOf(
+                ReferenceBook(id = "mo", name = "Mundorgel", abbreviation = "MO"),
+                ReferenceBook(id = "pl", name = "Pfadfinderlied", abbreviation = "PL")
+            )
+        )
+        val engineWithRefs = MeasurementEngine(fontMetrics, configWithRefs)
+
+        val songWithRefs = Song(
+            title = "With Refs",
+            references = mapOf("mo" to 42, "pl" to 17),
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+        val songWithoutRefs = Song(
+            title = "No Refs",
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+
+        val heightWith = engineWithRefs.measure(songWithRefs).totalHeightMm
+        val heightWithout = engineWithRefs.measure(songWithoutRefs).totalHeightMm
+
+        heightWith shouldBeGreaterThan heightWithout
+    }
+
+    @Test
+    fun `references do not add height when no reference books configured`() {
+        val songWithRefs = Song(
+            title = "With Refs",
+            references = mapOf("mo" to 42),
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+        val songWithoutRefs = Song(
+            title = "No Refs",
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+
+        // Default config has no reference books
+        val heightWith = engine.measure(songWithRefs).totalHeightMm
+        val heightWithout = engine.measure(songWithoutRefs).totalHeightMm
+
+        // Should be the same since no reference books are configured
+        heightWith shouldBe heightWithout
+    }
 }