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

DeepDrftModels/Enums/AudioFormat.cs

@@ -0,0 +1,24 @@
+namespace DeepDrftModels.Enums;
+
+/// <summary>
+/// The delivery format a listener requests for a track's audio (Phase 18). One <c>TrackEntity</c> /
+/// <c>EntryKey</c> addresses both renderings — "one source, multiple views" applied to delivery (C5).
+/// Lives here, not in the content library, because it is a cross-boundary contract: the API stream
+/// endpoint (18.3) parses it off the <c>?format=</c> query param, the WASM client (18.4 / 18.6) selects
+/// it, and the content-side resolver (18.2) resolves it to bytes — all three reference one enum.
+/// </summary>
+public enum AudioFormat
+{
+    /// <summary>
+    /// The existing source artifact in the <c>tracks</c> vault, served byte-for-byte with its real MIME
+    /// (WAV/MP3/FLAC — do not assume WAV). The universal, always-present rendering.
+    /// </summary>
+    Lossless,
+
+    /// <summary>
+    /// The derived low-data Ogg Opus 320 artifact in the <c>track-opus</c> vault (<c>audio/ogg</c>). A
+    /// best-effort derived artifact: when absent (not yet transcoded, or transcode failed) a request for
+    /// it falls back to <see cref="Lossless"/> rather than 404ing (C2).
+    /// </summary>
+    Opus
+}