add ReaderViewModel: observable view model wrapping AudioEngine, chapter offsets, synthesizer init

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

VorleserKit/Sources/AudioEngine/ReaderViewModel.swift

@@ -0,0 +1,76 @@
+import Foundation
+import Observation
+import BookParser
+import VorleserKit
+import class Synthesizer.Synthesizer
+import struct Synthesizer.VoicePack
+
+@Observable
+@MainActor
+public final class ReaderViewModel {
+	public private(set) var book: Book?
+	public private(set) var error: String?
+	public private(set) var synthesizer: Synthesizer?
+	public var selectedChapterIndex: Int = 0
+	public let engine = AudioEngine()
+
+	/// Character offset in the full book text where each chapter starts.
+	public private(set) var chapterOffsets: [Int] = []
+
+	public init() {}
+
+	public func loadBook(fileURL: URL, fileExists: Bool, lastPosition: Int, modelURL: URL?, voicesURL: URL?) async {
+		guard fileExists else {
+			self.error = "Book file is missing. Please re-import."
+			return
+		}
+		do {
+			let parsed = try BookParser.parse(url: fileURL)
+			self.book = parsed
+
+			// Build chapter offset table
+			var offsets: [Int] = []
+			var offset = 0
+			for chapter in parsed.chapters {
+				offsets.append(offset)
+				offset += chapter.text.count
+			}
+			self.chapterOffsets = offsets
+
+			// Restore chapter position
+			if lastPosition > 0,
+			   let (chIdx, _) = parsed.chapterAndLocalOffset(for: lastPosition) {
+				selectedChapterIndex = chIdx
+			}
+
+			// Init synthesizer
+			guard let modelURL, let voicesURL else {
+				error = "TTS model files not found in app bundle."
+				return
+			}
+			let voice = VoicePack.curated.first!
+			self.synthesizer = try Synthesizer(voice: voice, modelURL: modelURL, voicesURL: voicesURL)
+		} catch {
+			self.error = "Failed to load book: \(error)"
+		}
+	}
+
+	public func startPlayback(from offset: CharacterOffset) async throws {
+		guard let book, let synthesizer else { return }
+		try await engine.play(book: book, from: offset, using: synthesizer)
+	}
+
+	/// Returns the global character offset for a chapter index.
+	public func chapterOffset(for chapterIndex: Int) -> CharacterOffset {
+		guard chapterIndex < chapterOffsets.count else { return 0 }
+		return chapterOffsets[chapterIndex]
+	}
+
+	/// Returns the current sentence range in global character offsets, or nil if not playing.
+	public func currentSentenceRange() -> Range<CharacterOffset>? {
+		guard engine.state == .playing || engine.state == .synthesizing,
+		      let book else { return nil }
+		guard let idx = book.sentenceIndex(containing: engine.currentPosition) else { return nil }
+		return book.sentences[idx].range
+	}
+}