feat: support custom font files for song titles (Closes #4)

- Improve PdfFontMetrics: use canonical path for cache key, validate
  font file existence, use absolute paths for BaseFont.createFont
- Add font file path resolution in SongbookPipeline (relative to
  project directory)
- Add font file existence validation in Validator.validateConfig
- Add end-to-end tests: custom font loading, umlaut rendering,
  cache deduplication, missing file error
- Document custom font file usage in example songbook.yaml

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

parser/src/main/kotlin/de/pfadfinder/songbook/parser/Validator.kt

@@ -1,7 +1,9 @@
 package de.pfadfinder.songbook.parser
 
 import de.pfadfinder.songbook.model.BookConfig
+import de.pfadfinder.songbook.model.FontSpec
 import de.pfadfinder.songbook.model.Song
+import java.io.File
 
 data class ValidationError(val file: String?, val line: Int?, val message: String)
 
@@ -50,6 +52,27 @@ object Validator {
             if (outer <= 0) errors.add(ValidationError(file = null, line = null, message = "Outer margin must be greater than 0"))
         }
 
+        // Validate font files exist (paths should already be resolved to absolute by the pipeline)
+        validateFontFile(config.fonts.lyrics, "lyrics", errors)
+        validateFontFile(config.fonts.chords, "chords", errors)
+        validateFontFile(config.fonts.title, "title", errors)
+        validateFontFile(config.fonts.metadata, "metadata", errors)
+        validateFontFile(config.fonts.toc, "toc", errors)
+
         return errors
     }
+
+    private fun validateFontFile(font: FontSpec, fontRole: String, errors: MutableList<ValidationError>) {
+        val fontFile = font.file ?: return
+        val file = File(fontFile)
+        if (!file.exists()) {
+            errors.add(
+                ValidationError(
+                    file = null,
+                    line = null,
+                    message = "Font file for '$fontRole' not found: $fontFile"
+                )
+            )
+        }
+    }
 }

parser/src/test/kotlin/de/pfadfinder/songbook/parser/ConfigParserTest.kt

@@ -251,4 +251,21 @@ class ConfigParserTest {
         val config = ConfigParser.parse(yaml)
         config.book.title shouldBe "Test"
     }
+
+    @Test
+    fun `parse config with custom title font file only`() {
+        val yaml = """
+            book:
+              title: "Fraktur Test"
+            fonts:
+              title: { file: "./fonts/Fraktur.ttf", size: 16 }
+        """.trimIndent()
+        val config = ConfigParser.parse(yaml)
+        config.fonts.title.file shouldBe "./fonts/Fraktur.ttf"
+        config.fonts.title.size shouldBe 16f
+        config.fonts.title.family shouldBe "Helvetica"  // default family as fallback
+        // Other fonts should still use defaults
+        config.fonts.lyrics.file.shouldBeNull()
+        config.fonts.lyrics.family shouldBe "Helvetica"
+    }
 }

parser/src/test/kotlin/de/pfadfinder/songbook/parser/ValidatorTest.kt

@@ -206,4 +206,53 @@ class ValidatorTest {
         errors shouldHaveSize 1
         errors[0].file shouldContain "myfile.chopro"
     }
+
+    @Test
+    fun `missing font file produces validation error`() {
+        val config = BookConfig(
+            fonts = FontsConfig(
+                title = FontSpec(file = "/nonexistent/path/FrakturFont.ttf", size = 14f)
+            )
+        )
+        val errors = Validator.validateConfig(config)
+        errors shouldHaveSize 1
+        errors[0].message shouldContain "title"
+        errors[0].message shouldContain "not found"
+    }
+
+    @Test
+    fun `multiple missing font files produce multiple errors`() {
+        val config = BookConfig(
+            fonts = FontsConfig(
+                title = FontSpec(file = "/nonexistent/title.ttf", size = 14f),
+                lyrics = FontSpec(file = "/nonexistent/lyrics.ttf", size = 10f)
+            )
+        )
+        val errors = Validator.validateConfig(config)
+        errors shouldHaveSize 2
+    }
+
+    @Test
+    fun `config with no font files produces no font errors`() {
+        val config = BookConfig()  // all default built-in fonts
+        val errors = Validator.validateConfig(config)
+        errors.shouldBeEmpty()
+    }
+
+    @Test
+    fun `config with existing font file produces no error`() {
+        // Create a temporary file to simulate an existing font file
+        val tempFile = kotlin.io.path.createTempFile(suffix = ".ttf").toFile()
+        try {
+            val config = BookConfig(
+                fonts = FontsConfig(
+                    title = FontSpec(file = tempFile.absolutePath, size = 14f)
+                )
+            )
+            val errors = Validator.validateConfig(config)
+            errors.shouldBeEmpty()
+        } finally {
+            tempFile.delete()
+        }
+    }
 }