Add WavWriter: mono PCM16 RIFF writer for the C# probe

tests/01-record-play-cs/WavWriter.cs

@@ -0,0 +1,40 @@
+namespace RecordPlayProbe;
+
+internal static class WavWriter
+{
+    public static void Write(string path, ReadOnlySpan<short> samples, int sampleRate)
+    {
+        const short channels = 1;
+        const short bitsPerSample = 16;
+        const short pcmFormat = 1;
+
+        int byteRate = sampleRate * channels * bitsPerSample / 8;
+        short blockAlign = (short)(channels * bitsPerSample / 8);
+        int dataBytes = samples.Length * sizeof(short);
+        int riffChunkSize = 36 + dataBytes;
+
+        using var fs = new FileStream(path, FileMode.Create, FileAccess.Write);
+        using var bw = new BinaryWriter(fs);
+
+        // RIFF header
+        bw.Write(new[] { (byte)'R', (byte)'I', (byte)'F', (byte)'F' });
+        bw.Write(riffChunkSize);
+        bw.Write(new[] { (byte)'W', (byte)'A', (byte)'V', (byte)'E' });
+
+        // fmt chunk
+        bw.Write(new[] { (byte)'f', (byte)'m', (byte)'t', (byte)' ' });
+        bw.Write(16);              // fmt chunk size for PCM
+        bw.Write(pcmFormat);
+        bw.Write(channels);
+        bw.Write(sampleRate);
+        bw.Write(byteRate);
+        bw.Write(blockAlign);
+        bw.Write(bitsPerSample);
+
+        // data chunk
+        bw.Write(new[] { (byte)'d', (byte)'a', (byte)'t', (byte)'a' });
+        bw.Write(dataBytes);
+        foreach (short s in samples)
+            bw.Write(s);
+    }
+}