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/Services/UserFacingErrorsTests.cs

@@ -0,0 +1,120 @@
+using AmiReel.Services;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AmiReel.Tests.Services;
+
+[TestClass]
+public class UserFacingErrorsTests
+{
+    [TestMethod]
+    public void Summarize_WithBlankMessage_ReturnsGenericMessage()
+    {
+        // Arrange
+        Exception exception = new("   ");
+
+        // Act
+        string result = UserFacingErrors.Summarize(exception);
+
+        // Assert
+        Assert.AreEqual("An unexpected error occurred.", result);
+    }
+
+    [TestMethod]
+    public void Summarize_WithNonProcessExitMessage_ReturnsMessageUnchanged()
+    {
+        // Arrange
+        Exception exception = new("The output directory could not be created.");
+
+        // Act
+        string result = UserFacingErrors.Summarize(exception);
+
+        // Assert
+        Assert.AreEqual("The output directory could not be created.", result);
+    }
+
+    [TestMethod]
+    public void Summarize_WithProcessExitAndOnlyBoilerplateLines_ReturnsFirstLineOnly()
+    {
+        // Arrange
+        Exception exception = new(
+            "Process exited with code 1.\n" +
+            "ffmpeg version 6.0\n" +
+            "built with gcc\n" +
+            "Input #0, avi, from 'clip.avi':\n" +
+            "Duration: 00:01:00.00\n");
+
+        // Act
+        string result = UserFacingErrors.Summarize(exception);
+
+        // Assert
+        Assert.AreEqual("Process exited with code 1.", result);
+    }
+
+    [TestMethod]
+    public void Summarize_WithProcessExitAndRealErrorLines_IncludesTheRealErrorLines()
+    {
+        // Arrange
+        Exception exception = new(
+            "Process exited with code 1.\n" +
+            "ffmpeg version 6.0\n" +
+            "[h264_nvenc @ 0x1] Cannot load libnvidia-encode.so.1\n" +
+            "Error initializing output stream 0:0 -- Error while opening encoder\n");
+
+        // Act
+        string result = UserFacingErrors.Summarize(exception);
+
+        // Assert
+        StringAssert.Contains(result, "Process exited with code 1.");
+        StringAssert.Contains(result, "Cannot load libnvidia-encode.so.1");
+        StringAssert.Contains(result, "Error while opening encoder");
+    }
+
+    [TestMethod]
+    public void Summarize_WithMoreThanThreeErrorLines_KeepsOnlyTheLastThree()
+    {
+        // Arrange
+        Exception exception = new(
+            "Process exited with code 1.\n" +
+            "error line 1\n" +
+            "error line 2\n" +
+            "error line 3\n" +
+            "error line 4\n");
+
+        // Act
+        string result = UserFacingErrors.Summarize(exception);
+        string[] lines = result.Split(Environment.NewLine);
+
+        // Assert
+        Assert.AreEqual(4, lines.Length);
+        CollectionAssert.DoesNotContain(lines, "error line 1");
+        CollectionAssert.Contains(lines, "error line 4");
+    }
+
+    [TestMethod]
+    [DataRow("ffmpeg version 6.0")]
+    [DataRow("built with gcc 12")]
+    [DataRow("Input #0, avi, from 'clip.avi':")]
+    [DataRow("Duration: 00:01:00.00, start: 0.000000")]
+    [DataRow("Stream #0:0: Video: mjpeg")]
+    public void IsNoise_WithFfmpegBoilerplate_ReturnsTrue(string line)
+    {
+        // Act
+        bool result = UserFacingErrors.IsNoise(line);
+
+        // Assert
+        Assert.IsTrue(result);
+    }
+
+    [TestMethod]
+    [DataRow("Error while opening encoder for output stream")]
+    [DataRow("Cannot load libnvidia-encode.so.1")]
+    [DataRow("No such file or directory")]
+    public void IsNoise_WithRealErrorText_ReturnsFalse(string line)
+    {
+        // Act
+        bool result = UserFacingErrors.IsNoise(line);
+
+        // Assert
+        Assert.IsFalse(result);
+    }
+}