task 5: add AttributedStringBuilder for platform-aware font attributes

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

VorleserKit/Sources/BookParser/AttributedStringBuilder.swift

@@ -0,0 +1,121 @@
+import Foundation
+
+#if canImport(UIKit)
+import UIKit
+#elseif canImport(AppKit)
+import AppKit
+#endif
+
+/// Builds an NSAttributedString from plain text with formatting ranges.
+/// The resulting .string is character-for-character identical to the input text.
+enum AttributedStringBuilder {
+	struct FormattingRange {
+		enum Style {
+			case body
+			case bold
+			case italic
+			case heading(level: Int)
+			case paragraphStart
+		}
+		let range: NSRange
+		let style: Style
+	}
+
+	/// Build an attributed string from plain text and formatting ranges.
+	/// The plain text is used as-is — no characters are added or removed.
+	static func build(text: String, ranges: [FormattingRange]) -> NSAttributedString {
+		let result = NSMutableAttributedString(
+			string: text,
+			attributes: [
+				.font: bodyFont(),
+				.foregroundColor: textColor(),
+			]
+		)
+
+		for range in ranges {
+			switch range.style {
+			case .body:
+				break
+			case .bold:
+				result.addAttribute(.font, value: boldFont(), range: range.range)
+			case .italic:
+				result.addAttribute(.font, value: italicFont(), range: range.range)
+			case .heading(let level):
+				result.addAttribute(.font, value: headingFont(level: level), range: range.range)
+				let paragraphStyle = NSMutableParagraphStyle()
+				paragraphStyle.paragraphSpacingBefore = 16
+				result.addAttribute(.paragraphStyle, value: paragraphStyle, range: range.range)
+			case .paragraphStart:
+				let paragraphStyle = NSMutableParagraphStyle()
+				paragraphStyle.paragraphSpacingBefore = 8
+				result.addAttribute(.paragraphStyle, value: paragraphStyle, range: range.range)
+			}
+		}
+
+		return NSAttributedString(attributedString: result)
+	}
+
+	/// Build a plain attributed string with body font (no formatting ranges).
+	static func buildPlain(text: String) -> NSAttributedString {
+		NSAttributedString(
+			string: text,
+			attributes: [
+				.font: bodyFont(),
+				.foregroundColor: textColor(),
+			]
+		)
+	}
+
+	// MARK: - Platform fonts
+
+	#if canImport(UIKit)
+	private static func bodyFont() -> UIFont {
+		.preferredFont(forTextStyle: .body)
+	}
+
+	private static func boldFont() -> UIFont {
+		let descriptor = UIFont.preferredFont(forTextStyle: .body).fontDescriptor.withSymbolicTraits(.traitBold)!
+		return UIFont(descriptor: descriptor, size: 0)
+	}
+
+	private static func italicFont() -> UIFont {
+		let descriptor = UIFont.preferredFont(forTextStyle: .body).fontDescriptor.withSymbolicTraits(.traitItalic)!
+		return UIFont(descriptor: descriptor, size: 0)
+	}
+
+	private static func headingFont(level: Int) -> UIFont {
+		let style: UIFont.TextStyle = level <= 2 ? .title1 : .title3
+		let descriptor = UIFont.preferredFont(forTextStyle: style).fontDescriptor.withSymbolicTraits(.traitBold)!
+		return UIFont(descriptor: descriptor, size: 0)
+	}
+
+	private static func textColor() -> UIColor {
+		.label
+	}
+
+	#elseif canImport(AppKit)
+	private static func bodyFont() -> NSFont {
+		.preferredFont(forTextStyle: .body)
+	}
+
+	private static func boldFont() -> NSFont {
+		let body = NSFont.preferredFont(forTextStyle: .body)
+		return NSFontManager.shared.convert(body, toHaveTrait: .boldFontMask)
+	}
+
+	private static func italicFont() -> NSFont {
+		let body = NSFont.preferredFont(forTextStyle: .body)
+		return NSFontManager.shared.convert(body, toHaveTrait: .italicFontMask)
+	}
+
+	private static func headingFont(level: Int) -> NSFont {
+		let style: NSFont.TextStyle = level <= 2 ? .title1 : .title3
+		let heading = NSFont.preferredFont(forTextStyle: style)
+		return NSFontManager.shared.convert(heading, toHaveTrait: .boldFontMask)
+	}
+
+	private static func textColor() -> NSColor {
+		.textColor
+	}
+	#endif
+}