feat: segment playback slider with resolved YouTube chapters

Sources/App/YouTubeResolver.swift

@@ -7,6 +7,12 @@ struct YouTubeStream: Sendable {
     let headers: String
 }
 
+struct YouTubeChapter: Sendable {
+    let start: Double
+    let end: Double
+    let title: String?
+}
+
 struct YouTubeVideo: Sendable {
     let title: String
     let video: YouTubeStream
@@ -16,6 +22,19 @@ struct YouTubeVideo: Sendable {
     let webpageURL: URL
     let height: Int?
     let availableHeights: [Int]
+    let chapters: [YouTubeChapter]
+
+    func chapterBoundaries(duration: Double) -> [Double] {
+        guard duration.isFinite, duration > 0 else { return [] }
+        var boundaries = Set<Double>()
+        for chapter in chapters {
+            guard chapter.start.isFinite, chapter.end.isFinite,
+                  chapter.start >= 0, chapter.end > chapter.start, chapter.end <= duration else { continue }
+            if chapter.start > 0 { boundaries.insert(chapter.start) }
+            if chapter.end < duration { boundaries.insert(chapter.end) }
+        }
+        return boundaries.sorted()
+    }
 }
 
 private struct YouTubeResolutionError: LocalizedError {
@@ -135,7 +154,8 @@ enum YouTubeResolver {
             video: video, audio: audio, duration: duration,
             isLive: metadata.is_live == true || metadata.live_status == "is_live",
             webpageURL: try validateURL(webpageURL.absoluteString),
-            height: videoFormat.height, availableHeights: availableHeights
+            height: videoFormat.height, availableHeights: availableHeights,
+            chapters: metadata.chapters?.values ?? []
         )
     }
 
@@ -386,12 +406,38 @@ private struct YouTubeMetadata: Decodable {
     let formats: [YouTubeMetadata]?
     let http_headers: [String: String]?
     let requested_formats: [YouTubeMetadata]?
+    let chapters: YouTubeChapters?
 
     // Generic extractors may omit codec hints; FFmpeg probes the actual streams.
     var hasVideo: Bool { vcodec.map { !$0.isEmpty && $0 != "none" } ?? true }
     var hasAudio: Bool { acodec.map { !$0.isEmpty && $0 != "none" } ?? true }
 }
 
+private struct YouTubeChapters: Decodable {
+    let values: [YouTubeChapter]
+
+    private enum CodingKeys: String, CodingKey {
+        case start_time, end_time, title
+    }
+
+    init(from decoder: Decoder) {
+        guard var rows = try? decoder.unkeyedContainer() else {
+            values = []
+            return
+        }
+        var chapters: [YouTubeChapter] = []
+        while !rows.isAtEnd {
+            guard let rowDecoder = try? rows.superDecoder() else { break }
+            guard let row = try? rowDecoder.container(keyedBy: CodingKeys.self),
+                  let start = try? row.decode(Double.self, forKey: .start_time),
+                  let end = try? row.decode(Double.self, forKey: .end_time) else { continue }
+            chapters.append(YouTubeChapter(start: start, end: end,
+                title: try? row.decode(String.self, forKey: .title)))
+        }
+        values = chapters
+    }
+}
+
 private struct YouTubeExtractionOutput: Sendable {
     let stdout: Data
     let stderr: Data