fix(api): stream audio store path to eliminate whole-file buffering (OOM)

Processors now emit a ProcessedAudio plan with a streamed writer instead of a whole-file AudioBinary; vault writes stream via RegisterResourceStreamingAsync. Header parsing is bounded. Wave 2 (waveform/Opus) still re-reads the full file by design.

DeepDrftContent/Processors/FlacAudioProcessor.cs

@@ -12,7 +12,11 @@ public class FlacAudioProcessor
     private const double FallbackDuration = 180.0;
     private const int FallbackBitrate = 1411;
 
-    public async Task<AudioBinary?> ProcessFlacFileAsync(string filePath)
+    // STREAMINFO is mandatory and always the first metadata block, immediately after the 4-byte magic
+    // (data at offset 8, 34 bytes). A small prefix read covers it without loading the body.
+    private const long HeaderCap = 64 * 1024;
+
+    public async Task<ProcessedAudio?> ProcessFlacFileAsync(string filePath, CancellationToken cancellationToken = default)
     {
         if (!File.Exists(filePath))
         {
@@ -24,25 +28,21 @@ public class FlacAudioProcessor
             throw new ArgumentException("File must be a FLAC file", nameof(filePath));
         }
 
-        var buffer = await File.ReadAllBytesAsync(filePath);
-        var meta = ExtractFlacMetadata(buffer);
+        var fileLength = new FileInfo(filePath).Length;
+        var window = await AudioStoreStream.ReadPrefixAsync(filePath, HeaderCap, cancellationToken);
+        var meta = ExtractFlacMetadata(window, fileLength);
 
-        var parameters = new AudioBinaryParams(
-            Buffer: buffer,
-            Size: buffer.Length,
-            Extension: ".flac",
-            Duration: meta.Duration,
-            Bitrate: meta.Bitrate);
-
-        return new AudioBinary(parameters);
+        // FLAC is stored unmodified — passthrough the original bytes via a streamed disk-to-disk copy.
+        return ProcessedAudio.Passthrough(filePath, ".flac", meta.Duration, meta.Bitrate, fileLength);
     }
 
     /// <summary>
     /// Validates the <c>fLaC</c> magic and the leading STREAMINFO block, then computes duration from
     /// total-samples / sample-rate and average bitrate from file size. On any parse failure, logs a
-    /// warning and returns synthetic defaults — never throws.
+    /// warning and returns synthetic defaults — never throws. <paramref name="fileLength"/> is the true
+    /// file size (the header window may be shorter), used for the average-bitrate computation.
     /// </summary>
-    private static FlacMetadata ExtractFlacMetadata(byte[] buffer)
+    private static FlacMetadata ExtractFlacMetadata(byte[] buffer, long fileLength)
     {
         try
         {
@@ -84,7 +84,7 @@ public class FlacAudioProcessor
 
             var duration = (double)totalSamples / sampleRate;
             var bitrate = duration > 0
-                ? (int)(buffer.LongLength * 8L / (duration * 1000))
+                ? (int)(fileLength * 8L / (duration * 1000))
                 : FallbackBitrate;
 
             return new FlacMetadata { Duration = duration, Bitrate = bitrate };