feat(engine): let CreateSender carry the output frame rate (N/D)

The sender previously had no way to learn the configured frame rate, so
NdiInteropPInvoke hardcoded 60000/1001 (59.94) into every outgoing frame
regardless of the operator's target — silently defeating the framerate
half of "normalization." CreateSender now takes frameRateN/frameRateD,
defaulted to 60000/1001 so existing call sites and the fake compile
unchanged. The sender pins this onto its handle and stamps it on each
frame.

src/Dragon-ISO.Engine/Interop/INdiInterop.cs

@@ -47,14 +47,22 @@ public interface INdiInterop
     // ----- Send -----
 
     /// <summary>
-    /// Creates an NDI sender broadcasting on the given groups.
+    /// Creates an NDI sender broadcasting on the given groups at the given frame rate.
     /// </summary>
     /// <param name="outputName">The NDI source name segment (the part inside parens).</param>
     /// <param name="groups">
     /// Comma-separated list of NDI group names to broadcast on. <c>null</c> or empty
     /// uses the NDI default (<c>"Public"</c>).
     /// </param>
-    NdiSenderHandle CreateSender(string outputName, string? groups = null);
+    /// <param name="frameRateN">
+    /// Frame-rate numerator advertised on every frame the sender emits (the NDI
+    /// <c>frame_rate_N</c>). Defaults to 60000 (the numerator of 59.94).
+    /// </param>
+    /// <param name="frameRateD">
+    /// Frame-rate denominator (the NDI <c>frame_rate_D</c>). Defaults to 1001, so the
+    /// default pair 60000/1001 is exactly 59.94 fps. Must be positive.
+    /// </param>
+    NdiSenderHandle CreateSender(string outputName, string? groups = null, int frameRateN = 60000, int frameRateD = 1001);
 
     void SendFrame(NdiSenderHandle sender, ProcessedFrame frame);