feature: Phase 18.1 — derive Opus 320 + seek-index sidecar at ingest

Background-job transcode (ffmpeg/libopus) after source store; pure C# Ogg
walker builds the 0.5s-bucketed granule→byte seek index + captures the
OpusHead/OpusTags setup header into a per-track sidecar in a new track-opus
vault. Best-effort, additive, regenerated on replace-audio.

DeepDrftContent/Constants/VaultConstants.cs

@@ -28,4 +28,13 @@ public static class VaultConstants
     /// The datum resolution is duration-derived (≈333 samples/sec, see <c>WaveformResolution</c>).
     /// </summary>
     public const string TrackWaveforms = "track-waveforms";
+
+    /// <summary>
+    /// Vault name for the derived low-data Ogg Opus artifacts, keyed by the track's EntryKey (Phase 18,
+    /// S2). Holds two entries per track: the Opus audio bytes (<c>.opus</c>) and the combined setup-header
+    /// + granule→byte seek-index sidecar (<c>.opusidx</c>). Both are best-effort derived artifacts —
+    /// regenerable, and a track without them still plays losslessly. Distinct from the source <c>tracks</c>
+    /// vault so the source means exactly one thing (mirrors the <c>track-waveforms</c> precedent).
+    /// </summary>
+    public const string TrackOpus = "track-opus";
 }