feat: live TV time-shift

lib/models/livetv_capture_buffer.dart

@@ -0,0 +1,51 @@
+/// Represents the seekable capture buffer for a live TV transcode session.
+///
+/// Extracted from the `TranscodeSession` element in the tune response:
+/// - [startedAt] = `timeStamp` (epoch seconds, the buffer origin)
+/// - [seekStartSeconds] = `minOffsetAvailable` (earliest seekable offset)
+/// - [seekEndSeconds] = `maxOffsetAvailable` (latest seekable offset / live edge)
+class CaptureBuffer {
+  final double startedAt;
+  final double seekStartSeconds;
+  final double seekEndSeconds;
+
+  const CaptureBuffer({
+    required this.startedAt,
+    required this.seekStartSeconds,
+    required this.seekEndSeconds,
+  });
+
+  /// Absolute epoch second of the earliest seekable point.
+  int get seekableStartEpoch => (startedAt + seekStartSeconds).round();
+
+  /// Absolute epoch second of the latest seekable point (≈ live edge).
+  int get seekableEndEpoch => (startedAt + seekEndSeconds).round();
+
+  /// Duration of the seekable range in seconds.
+  int get seekableDurationSeconds => (seekEndSeconds - seekStartSeconds).round();
+
+  /// Parse from a TranscodeSession JSON map. Returns null if required fields are missing.
+  /// Values may be num or String depending on whether the server returned JSON or XML.
+  static CaptureBuffer? fromTranscodeSession(Map<String, dynamic> session) {
+    final timeStamp = _parseDouble(session['timeStamp']);
+    final minOffset = _parseDouble(session['minOffsetAvailable']);
+    final maxOffset = _parseDouble(session['maxOffsetAvailable']);
+    if (timeStamp == null || minOffset == null || maxOffset == null) return null;
+    return CaptureBuffer(
+      startedAt: timeStamp,
+      seekStartSeconds: minOffset,
+      seekEndSeconds: maxOffset,
+    );
+  }
+
+  static double? _parseDouble(dynamic value) {
+    if (value is num) return value.toDouble();
+    if (value is String) return double.tryParse(value);
+    return null;
+  }
+
+  @override
+  String toString() =>
+      'CaptureBuffer(startedAt: $startedAt, seek: $seekStartSeconds..$seekEndSeconds, '
+      'range: $seekableStartEpoch..$seekableEndEpoch)';
+}