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/AppSettingsTests.cs

@@ -0,0 +1,107 @@
+using AmiReel.Models;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AmiReel.Tests.Models;
+
+[TestClass]
+public class AppSettingsTests
+{
+    [TestMethod]
+    public void Default_SetsFallbackOutputFolderAndSensibleDefaults()
+    {
+        // Arrange
+        const string outputFolder = @"C:\Videos";
+
+        // Act
+        AppSettings settings = AppSettings.Default(outputFolder);
+
+        // Assert
+        Assert.AreEqual(outputFolder, settings.OutputFolder);
+        Assert.AreEqual("Dark", settings.Theme);
+        Assert.AreEqual("Auto", settings.Encoder);
+        Assert.IsTrue(settings.ShouldMoveSourcesToOriginals);
+    }
+
+    [TestMethod]
+    public void ShouldMoveSourcesToOriginals_WhenFlagIsNull_DefaultsToTrue()
+    {
+        // Arrange
+        AppSettings settings = AppSettings.Default(@"C:\Videos") with { MoveSourcesToOriginals = null };
+
+        // Act
+        bool result = settings.ShouldMoveSourcesToOriginals;
+
+        // Assert
+        Assert.IsTrue(result);
+    }
+
+    [TestMethod]
+    public void ShouldMoveSourcesToOriginals_WhenFlagIsExplicitlyFalse_ReturnsFalse()
+    {
+        // Arrange
+        AppSettings settings = AppSettings.Default(@"C:\Videos") with { MoveSourcesToOriginals = false };
+
+        // Act
+        bool result = settings.ShouldMoveSourcesToOriginals;
+
+        // Assert
+        Assert.IsFalse(result);
+    }
+
+    [TestMethod]
+    public void Normalize_WithBlankOutputFolder_UsesFallback()
+    {
+        // Arrange
+        AppSettings settings = AppSettings.Default("") with { OutputFolder = "   " };
+
+        // Act
+        AppSettings normalized = settings.Normalize(@"C:\Fallback");
+
+        // Assert
+        Assert.AreEqual(@"C:\Fallback", normalized.OutputFolder);
+    }
+
+    [TestMethod]
+    public void Normalize_WithBlankTimingValues_RestoresDefaults()
+    {
+        // Arrange
+        AppSettings settings = AppSettings.Default(@"C:\Videos") with
+        {
+            TrimStart = "",
+            FadeSeconds = "  ",
+            EndCardHoldSeconds = "",
+            ThumbnailInterval = "",
+        };
+
+        // Act
+        AppSettings normalized = settings.Normalize(@"C:\Videos");
+
+        // Assert
+        Assert.AreEqual("4.414", normalized.TrimStart);
+        Assert.AreEqual("3", normalized.FadeSeconds);
+        Assert.AreEqual("4", normalized.EndCardHoldSeconds);
+        Assert.AreEqual("10", normalized.ThumbnailInterval);
+    }
+
+    [TestMethod]
+    public void Normalize_WithPopulatedValues_KeepsThemUnchanged()
+    {
+        // Arrange
+        AppSettings settings = AppSettings.Default(@"C:\Videos") with
+        {
+            TrimStart = "1.5",
+            FadeSeconds = "2.5",
+            Theme = "Light",
+            Encoder = "NvidiaNvenc",
+        };
+
+        // Act
+        AppSettings normalized = settings.Normalize(@"C:\Videos");
+
+        // Assert
+        Assert.AreEqual("1.5", normalized.TrimStart);
+        Assert.AreEqual("2.5", normalized.FadeSeconds);
+        Assert.AreEqual("Light", normalized.Theme);
+        Assert.AreEqual("NvidiaNvenc", normalized.Encoder);
+    }
+}