feat: support inline images within song pages (Closes #2)

Add {image: path} directive to embed images at any position within a song's
sections. SongLine gains an optional imagePath field; when set, the line
represents an inline image rather than chord/lyric content. The renderer
scales and centers images within the content width. MeasurementEngine
reserves 40mm height per inline image for layout calculations.

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

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

@@ -205,8 +205,14 @@ class PdfBookRenderer : BookRenderer {
 
             // Render lines
             for (line in section.lines) {
-                val height = chordLyricRenderer.renderLine(cb, line, leftMargin, y, contentWidth)
-                y -= height + 1f  // 1pt gap between lines
+                val imgPath = line.imagePath
+                if (imgPath != null) {
+                    // Render inline image
+                    y -= renderInlineImage(cb, imgPath, leftMargin, y, contentWidth)
+                } else {
+                    val height = chordLyricRenderer.renderLine(cb, line, leftMargin, y, contentWidth)
+                    y -= height + 1f  // 1pt gap between lines
+                }
             }
 
             // End repeat marker
@@ -446,6 +452,37 @@ class PdfBookRenderer : BookRenderer {
         return lines
     }
 
+    /**
+     * Renders an inline image within a song page at the given position.
+     * Returns the total height consumed in PDF points.
+     */
+    private fun renderInlineImage(
+        cb: PdfContentByte,
+        imagePath: String,
+        leftMargin: Float,
+        y: Float,
+        contentWidth: Float
+    ): Float {
+        try {
+            val img = Image.getInstance(imagePath)
+            // Scale to fit within content width, max height 40mm (~113 points)
+            val maxHeight = 40f / 0.3528f  // 40mm in points
+            img.scaleToFit(contentWidth * 0.8f, maxHeight)
+
+            // Center horizontally
+            val imgX = leftMargin + (contentWidth - img.scaledWidth) / 2
+            val imgY = y - img.scaledHeight - 3f  // 3pt gap above
+
+            img.setAbsolutePosition(imgX, imgY)
+            cb.addImage(img)
+
+            return img.scaledHeight + 6f  // image height + gaps above/below
+        } catch (_: Exception) {
+            // If image can't be loaded, consume minimal space
+            return 5f
+        }
+    }
+
     private fun renderFillerImage(document: Document, imagePath: String, pageSize: Rectangle) {
         try {
             val img = Image.getInstance(imagePath)