feat: replace ?offset= seek with HTTP Range streaming across API, proxy, and client

- API: enableRangeProcessing true on no-offset FileStream path
- Proxy: transparent Range relay, forwards 206/416/Content-Range verbatim
- TrackMediaClient: Range: bytes=X- replaces ?offset=X; response disposed via TrackMediaResponse
- StreamDecoder: reinitializeForRangeContinuation retains wavHeader, counts raw PCM against 206 Content-Length
- AudioPlayer: seekBeyondBuffer adds headerSize for file-absolute offset; duration guard prevents continuation overwriting full-track duration
- StreamingAudioPlayerService: seek guard corrected to >= 0 (file-absolute offset contract)

DeepDrftPublic/Interop/audio/AudioPlayer.ts

@@ -321,18 +321,28 @@ export class AudioPlayer {
      */
     private seekBeyondBuffer(position: number): AudioResult {
         try {
-            const byteOffset = this.streamDecoder.calculateByteOffset(position);
+            const audioOffset = this.streamDecoder.calculateByteOffset(position);
             // 0 is a valid offset (seek to start of audio data). Only a negative result
             // indicates calculation failure — typically a missing/unparsed WAV header.
-            if (byteOffset < 0) {
+            if (audioOffset < 0) {
                 return { success: false, error: 'Cannot calculate byte offset' };
             }
 
-            // Signal that C# needs to request new stream from offset
+            // The Range request is file-absolute: byte position from the start of the
+            // file on disk, header included. calculateByteOffset returns an audio-data-
+            // relative offset, so add headerSize to land on the right byte. (The old
+            // ?offset= contract was audio-relative; the server added the header itself.)
+            const header = this.streamDecoder.getWavHeader();
+            if (!header) {
+                return { success: false, error: 'Cannot calculate byte offset' };
+            }
+            const fileOffset = header.headerSize + audioOffset;
+
+            // Signal that C# needs to request a new stream from this file-absolute offset
             return {
                 success: true,
                 seekBeyondBuffer: true,
-                byteOffset: byteOffset
+                byteOffset: fileOffset
             };
         } catch (error) {
             return { success: false, error: (error as Error).message };
@@ -368,8 +378,10 @@ export class AudioPlayer {
             this.scheduler.clearForSeek();
             this.scheduler.setPlaybackOffset(seekPosition);
 
-            // Reinitialize decoder for new stream
-            this.streamDecoder.reinitializeForOffset(totalStreamLength);
+            // Reinitialize decoder for the Range-continuation stream. totalStreamLength
+            // here is the 206 Content-Length (range start → EOF), not the full file size —
+            // the decoder uses it to detect stream-complete against raw audio bytes.
+            this.streamDecoder.reinitializeForRangeContinuation(totalStreamLength);
 
             // Update state
             this.pausePosition = seekPosition;