Add unit tests, dedupe FFmpeg output filtering, harden settings records

Implements the suggestions from the last review pass:

- Add AmiReel.Tests (MSTest), covering the pure logic in Models/ and
  Services/: SupportedVideoFormats, AppSettings normalization,
  UserFacingErrors.Summarize, the new FfmpegProgressParser/
  FfmpegOutputFilter, RenderPipeline.Validate, BuildProgressMessage, and
  MoveSourceVideos. 59 tests, all passing. UI code-behind and anything that
  spawns an actual FFmpeg process are left to manual/integration testing.
  Exclude AmiReel.Tests\**\*.cs from AmiReel.csproj's default item glob —
  it's a subfolder of the app project now, so without the exclude the app
  itself was compiling the MSTest-only test files.

- Extract the FFmpeg version/library-banner boilerplate list that
  ProcessRunner (live log filter) and UserFacingErrors (error summarizer)
  had each duplicated into a shared FfmpegOutputFilter.IsBoilerplateLine;
  each caller keeps its own remaining context-specific checks on top.
  Also extract ProcessRunner's line-parsing regexes into a standalone
  FfmpegProgressParser so it's directly unit-testable without spawning a
  process.

- Convert AppSettings and RenderSettings from positional record
  constructors to named `required` init properties. Both records had
  runs of same-typed consecutive parameters (three string timing fields
  in AppSettings; three doubles then five ints in RenderSettings) that a
  positional constructor would let get silently transposed at a call site
  without the compiler catching it. Update the two call sites
  (MainPage.xaml.cs) to object-initializer syntax.

- Add Properties/AssemblyInfo.cs with InternalsVisibleTo("AmiReel.Tests")
  and make Validate/BuildProgressMessage/IsNoise internal so tests can
  reach them directly instead of only through process-spawning entry
  points.

- README: document `dotnet test`, and note that Package.appxmanifest's
  Identity is a local-dev placeholder that needs a real publisher/cert
  before MSIX distribution.

Verified: dotnet build (solution + test project) is 0 warnings/errors,
dotnet test is 59/59 passing, and the app still launches unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

AmiReel.Tests/Models/SupportedVideoFormatsTests.cs

@@ -0,0 +1,69 @@
+using AmiReel.Models;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AmiReel.Tests.Models;
+
+[TestClass]
+public class SupportedVideoFormatsTests
+{
+    [TestMethod]
+    [DataRow(@"C:\videos\clip.avi")]
+    [DataRow(@"C:\videos\clip.mp4")]
+    [DataRow(@"C:\videos\clip.MOV")]
+    [DataRow(@"C:\videos\clip.mkv")]
+    [DataRow(@"C:\videos\clip.webm")]
+    public void IsSupported_WithAcceptedExtension_ReturnsTrue(string path)
+    {
+        // Act
+        bool result = SupportedVideoFormats.IsSupported(path);
+
+        // Assert
+        Assert.IsTrue(result);
+    }
+
+    [TestMethod]
+    [DataRow(@"C:\documents\report.pdf")]
+    [DataRow(@"C:\images\photo.png")]
+    [DataRow(@"C:\audio\track.mp3")]
+    public void IsSupported_WithUnsupportedExtension_ReturnsFalse(string path)
+    {
+        // Act
+        bool result = SupportedVideoFormats.IsSupported(path);
+
+        // Assert
+        Assert.IsFalse(result);
+    }
+
+    [TestMethod]
+    public void IsSupported_IsCaseInsensitive()
+    {
+        // Arrange
+        const string lower = @"C:\videos\clip.avi";
+        const string upper = @"C:\videos\clip.AVI";
+
+        // Act & Assert
+        Assert.IsTrue(SupportedVideoFormats.IsSupported(lower));
+        Assert.IsTrue(SupportedVideoFormats.IsSupported(upper));
+    }
+
+    [TestMethod]
+    public void IsSupported_WithNoExtension_ReturnsFalse()
+    {
+        // Act
+        bool result = SupportedVideoFormats.IsSupported(@"C:\videos\clip");
+
+        // Assert
+        Assert.IsFalse(result);
+    }
+
+    [TestMethod]
+    public void PickerFilter_ContainsEveryExtensionAsAGlobPattern()
+    {
+        // Act
+        string filter = SupportedVideoFormats.PickerFilter;
+
+        // Assert
+        foreach (string extension in SupportedVideoFormats.Extensions)
+            StringAssert.Contains(filter, "*" + extension);
+    }
+}