feat(engine): expose framerate as an exact rational (N/D)

FramerateHz collapses 59.94 etc. to a lossy double. The NDI sender needs
the exact frame_rate_N / frame_rate_D rational the broadcast standard
defines (59.94 = 60000/1001, not 59.94/1). Adds a FrameRate (N, D)
property mapping each TargetFramerate to its canonical rational so the
sender can advertise the correct cadence instead of a rounded value.
Pure + unit-testable; no behavior change on its own.

src/Dragon-ISO.Engine/Domain/FrameProcessingSettings.cs

@@ -23,6 +23,27 @@ public sealed record FrameProcessingSettings(
         _ => throw new InvalidOperationException($"Unknown framerate: {Framerate}")
     };
 
+    /// <summary>
+    /// Returns the framerate as an exact rational (numerator / denominator), as
+    /// required by the NDI sender's <c>frame_rate_N</c> / <c>frame_rate_D</c> and
+    /// by container muxers. The broadcast fractional rates are 1000/1001 pulldowns
+    /// of their integer parents (e.g. 59.94 = 60000/1001), so we must carry the
+    /// rational rather than a rounded <see cref="FramerateHz"/> double — advertising
+    /// 60/1 for a 59.94 stream causes drift and A/V desync downstream.
+    /// </summary>
+    public (int Numerator, int Denominator) FrameRate => Framerate switch
+    {
+        TargetFramerate.Fps23_976 => (24000, 1001),
+        TargetFramerate.Fps24     => (24, 1),
+        TargetFramerate.Fps25     => (25, 1),
+        TargetFramerate.Fps29_97  => (30000, 1001),
+        TargetFramerate.Fps30     => (30, 1),
+        TargetFramerate.Fps50     => (50, 1),
+        TargetFramerate.Fps59_94  => (60000, 1001),
+        TargetFramerate.Fps60     => (60, 1),
+        _ => throw new InvalidOperationException($"Unknown framerate: {Framerate}")
+    };
+
     /// <summary>Returns the resolution as (width, height).</summary>
     public (int Width, int Height) ResolutionSize => Resolution switch
     {