feat(pipeline): add SolidFrameRenderer slate and IFrameScaler/PassthroughFrameScaler

src/TeamsISO.Engine/Pipeline/IFrameScaler.cs

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+using TeamsISO.Engine.Domain;
+
+namespace TeamsISO.Engine.Pipeline;
+
+public interface IFrameScaler
+{
+    ProcessedFrame Scale(RawFrame source, int targetWidth, int targetHeight, AspectMode aspect, long timestampTicks);
+}

src/TeamsISO.Engine/Pipeline/PassthroughFrameScaler.cs

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+using TeamsISO.Engine.Domain;
+
+namespace TeamsISO.Engine.Pipeline;
+
+/// <summary>
+/// Phase A scaler. Copies the source frame's pixel buffer through unchanged and tags the
+/// output with the requested target dimensions. Real scaling is added in Phase B against libyuv.
+/// </summary>
+public sealed class PassthroughFrameScaler : IFrameScaler
+{
+    public ProcessedFrame Scale(RawFrame source, int targetWidth, int targetHeight, AspectMode aspect, long timestampTicks)
+    {
+        return new ProcessedFrame(
+            Width: targetWidth,
+            Height: targetHeight,
+            TimestampTicks: timestampTicks,
+            Pixels: source.Pixels,
+            Format: source.Format == PixelFormat.Bgra ? PixelFormat.Bgra : PixelFormat.Bgra);
+    }
+}

src/TeamsISO.Engine/Pipeline/SolidFrameRenderer.cs

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+namespace TeamsISO.Engine.Pipeline;
+
+/// <summary>
+/// Generates a solid-color BGRA frame for use as a "no signal" slate.
+/// </summary>
+public sealed class SolidFrameRenderer
+{
+    public ProcessedFrame Render(int width, int height, byte b, byte g, byte r, byte a, long timestampTicks)
+    {
+        var pixels = new byte[width * height * 4];
+        for (var i = 0; i < pixels.Length; i += 4)
+        {
+            pixels[i + 0] = b;
+            pixels[i + 1] = g;
+            pixels[i + 2] = r;
+            pixels[i + 3] = a;
+        }
+        return new ProcessedFrame(width, height, timestampTicks, pixels, PixelFormat.Bgra);
+    }
+}

src/tests/TeamsISO.Engine.Tests/Pipeline/SolidFrameRendererTests.cs

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+using TeamsISO.Engine.Pipeline;
+
+namespace TeamsISO.Engine.Tests.Pipeline;
+
+public class SolidFrameRendererTests
+{
+    [Fact]
+    public void Render_ProducesBgraFrameOfTargetSize_FilledWithColor()
+    {
+        var renderer = new SolidFrameRenderer();
+        var frame = renderer.Render(width: 1920, height: 1080, b: 0x80, g: 0x80, r: 0x80, a: 0xFF, timestampTicks: 12345);
+
+        frame.Width.Should().Be(1920);
+        frame.Height.Should().Be(1080);
+        frame.Format.Should().Be(PixelFormat.Bgra);
+        frame.Pixels.Length.Should().Be(1920 * 1080 * 4);
+        frame.TimestampTicks.Should().Be(12345);
+
+        var span = frame.Pixels.Span;
+        span[0].Should().Be(0x80); span[1].Should().Be(0x80); span[2].Should().Be(0x80); span[3].Should().Be(0xFF);
+        var last = span.Length - 4;
+        span[last].Should().Be(0x80); span[last + 3].Should().Be(0xFF);
+    }
+}