Add server-side waveform loudness profiling on track upload

ILoudnessAlgorithm strategy (RmsLoudnessAlgorithm first impl), WaveformProfileService
stores quantized byte[] sidecar in new MediaFileVault (profiles vault), wired into
UnifiedTrackService.UploadAsync; failure is logged and swallowed. WaveformProfileDto
and WaveformProfileOptions in shared projects.

DeepDrftContent/Processors/AudioProcessor.cs

@@ -45,6 +45,55 @@ public class AudioProcessor
         }
     }
 
+    /// <summary>
+    /// Extracts the raw PCM data region and format parameters from a WAV buffer, reusing the
+    /// same chunk-walk and validation as metadata extraction. Returns null if the buffer is not
+    /// a valid PCM WAV (callers treat a null as "no profile computable" and continue) — unlike
+    /// <see cref="ExtractWavMetadata"/>, this does NOT fall back to synthetic defaults, because a
+    /// loudness profile over fabricated silence would be misleading.
+    /// </summary>
+    public PcmData? TryExtractPcm(ReadOnlySpan<byte> buffer)
+    {
+        // Copy the span to an array so the existing array-based parsers can be reused. The PCM
+        // slice returned is a view over this array (no second copy of the data region).
+        var bytes = buffer.ToArray();
+
+        var validation = ValidateWavStructure(bytes);
+        if (!validation.IsValid)
+        {
+            return null;
+        }
+
+        WavMetadata metadata;
+        try
+        {
+            metadata = ParseWavMetadata(bytes, validation);
+            ValidateAudioParameters(metadata);
+        }
+        catch
+        {
+            return null;
+        }
+
+        // Data bytes begin 8 past the "data" chunk id (4 id + 4 size). Clamp the declared size to
+        // what is actually present — some encoders write a size that overshoots the file.
+        var dataStart = validation.DataChunkPos + 8;
+        if (dataStart > bytes.Length)
+        {
+            return null;
+        }
+
+        var available = bytes.Length - dataStart;
+        var dataLength = Math.Min(metadata.DataSize, available);
+        if (dataLength <= 0)
+        {
+            return null;
+        }
+
+        var pcm = new ReadOnlyMemory<byte>(bytes, dataStart, dataLength);
+        return new PcmData(pcm, metadata.Channels, metadata.SampleRate, metadata.BitsPerSample);
+    }
+
     /// <summary>
     /// Extracts metadata from WAV file buffer with comprehensive validation
     /// </summary>
@@ -268,4 +317,18 @@ public class AudioProcessor
         public int FmtChunkPos { get; set; }
         public int DataChunkPos { get; set; }
     }
-}
+}
+
+/// <summary>
+/// The raw PCM sample region of a WAV plus the format parameters needed to interpret it.
+/// <see cref="Pcm"/> is a view over the decoded buffer — the data chunk only, header excluded.
+/// </summary>
+/// <param name="Pcm">The PCM sample bytes (interleaved by channel, little-endian).</param>
+/// <param name="Channels">Number of interleaved channels.</param>
+/// <param name="SampleRate">Samples per second.</param>
+/// <param name="BitsPerSample">Bit depth per sample (8, 16, 24, or 32).</param>
+public readonly record struct PcmData(
+    ReadOnlyMemory<byte> Pcm,
+    int Channels,
+    int SampleRate,
+    int BitsPerSample);