cache Book.sentences as stored let property, add sentencesAreCached test

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

VorleserKit/Sources/BookParser/Book.swift

@@ -6,16 +6,14 @@ public struct Book: Sendable {
 	public let title: String
 	public let author: String?
 	public let chapters: [Chapter]
+	public let sentences: [Sentence]
 
 	public init(id: UUID = UUID(), title: String, author: String?, chapters: [Chapter]) {
 		self.id = id
 		self.title = title
 		self.author = author
 		self.chapters = chapters
-	}
 
-	/// All sentences across all chapters, with global character offsets.
-	public var sentences: [Sentence] {
 		var result: [Sentence] = []
 		var offset: CharacterOffset = 0
 		for chapter in chapters {
@@ -23,13 +21,12 @@ public struct Book: Sendable {
 			result.append(contentsOf: chapterSentences)
 			offset += chapter.text.count
 		}
-		return result
+		self.sentences = result
 	}
 
 	/// Returns the sentence index containing the given global character offset.
 	public func sentenceIndex(containing offset: CharacterOffset) -> Int? {
-		let allSentences = sentences
-		return allSentences.firstIndex { $0.range.contains(offset) }
+		sentences.firstIndex { $0.range.contains(offset) }
 	}
 
 	/// Maps a global character offset to (chapter index, local offset within chapter).

VorleserKit/Tests/BookParserTests/BookTests.swift

@@ -54,4 +54,12 @@ struct BookCharacterAddressingTests {
 	@Test func outOfRangeReturnsNil() {
 		#expect(book.chapterAndLocalOffset(for: 9999) == nil)
 	}
+
+	@Test func sentencesAreCached() {
+		let s1 = book.sentences
+		let s2 = book.sentences
+		#expect(s1.count == s2.count)
+		#expect(s1[0].text == s2[0].text)
+		#expect(s1[0].range == s2[0].range)
+	}
 }