feature: Opus format resolution + sidecar lookup contract (Phase 18.2)

DeepDrftContent/Processors/Opus/ResolvedAudio.cs

@@ -0,0 +1,23 @@
+using DeepDrftContent.FileDatabase.Models;
+using DeepDrftModels.Enums;
+
+namespace DeepDrftContent.Processors.Opus;
+
+/// <summary>
+/// The outcome of resolving a track + requested <see cref="AudioFormat"/> to a concrete artifact
+/// (Phase 18.2). Carries the bytes, the content-type that matches <em>what was actually returned</em>,
+/// and the format actually served — which may differ from the requested one when the C2 fallback fires
+/// (Opus requested, no Opus artifact → the lossless artifact + its content-type). The delivery layer
+/// (18.3) sets the response <c>Content-Type</c> from <see cref="ContentType"/> so the eventual decoder
+/// picks the right decoder for the bytes it receives, not the bytes the listener asked for.
+/// </summary>
+/// <param name="Audio">The resolved audio artifact (never null when a resolution succeeds).</param>
+/// <param name="ContentType">The MIME type of <paramref name="Audio"/> (e.g. <c>audio/ogg</c> for Opus,
+/// or the source's real MIME for lossless).</param>
+/// <param name="ResolvedFormat">The format actually returned. Equal to the requested format on a direct
+/// hit; <see cref="AudioFormat.Lossless"/> when an Opus request fell back.</param>
+public sealed record ResolvedAudio(AudioBinary Audio, string ContentType, AudioFormat ResolvedFormat)
+{
+    /// <summary>True when an Opus request was served the lossless artifact because no Opus existed (C2).</summary>
+    public bool DidFallBack(AudioFormat requested) => requested != ResolvedFormat;
+}