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>

buildSrc/src/main/kotlin/songbook-conventions.gradle.kts

@@ -2,8 +2,15 @@ plugins {
     kotlin("jvm")
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
+}
+
 kotlin {
-    jvmToolchain(21)
+    compilerOptions {
+        jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_21)
+    }
 }
 
 tasks.withType<Test> {