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>

songbook.yaml

@@ -14,6 +14,11 @@ fonts:
   title:     { family: "Helvetica", size: 14 }
   metadata:  { family: "Helvetica", size: 8 }
   toc:       { family: "Helvetica", size: 9 }
+  # To use a custom font file (e.g. Fraktur/Blackletter for titles):
+  # title:   { file: "./fonts/FrakturFont.ttf", size: 16 }
+  # The file path is relative to the project directory.
+  # Supported formats: .ttf, .otf
+  # Custom fonts are embedded in the PDF and support Unicode (including umlauts).
 
 layout:
   margins: { top: 15, bottom: 15, inner: 20, outer: 12 }