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/FfmpegOutputFilterTests.cs

@@ -0,0 +1,48 @@
+using AmiReel.Services;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AmiReel.Tests.Services;
+
+[TestClass]
+public class FfmpegOutputFilterTests
+{
+    [TestMethod]
+    [DataRow("ffmpeg version 6.0-full_build-www.gyan.dev")]
+    [DataRow("built with gcc 12.2.0")]
+    [DataRow("configuration: --enable-gpl")]
+    [DataRow("libavutil      58.  2.100 / 58.  2.100")]
+    [DataRow("Input #0, avi, from 'clip.avi':")]
+    [DataRow("Output #0, mp4, to 'out.mp4':")]
+    [DataRow("Stream #0:0: Video: mjpeg")]
+    [DataRow("Duration: 00:01:00.00, start: 0.000000, bitrate: 128 kb/s")]
+    [DataRow("video:1024kB audio:128kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.5%")]
+    public void IsBoilerplateLine_WithKnownFfmpegBanner_ReturnsTrue(string line)
+    {
+        // Act
+        bool result = FfmpegOutputFilter.IsBoilerplateLine(line);
+
+        // Assert
+        Assert.IsTrue(result);
+    }
+
+    [TestMethod]
+    [DataRow("Error while opening encoder for output stream #0:0")]
+    [DataRow("Cannot load nvcuda.dll")]
+    [DataRow("Unknown encoder 'h264_nvenc'")]
+    public void IsBoilerplateLine_WithRealErrorText_ReturnsFalse(string line)
+    {
+        // Act
+        bool result = FfmpegOutputFilter.IsBoilerplateLine(line);
+
+        // Assert
+        Assert.IsFalse(result);
+    }
+
+    [TestMethod]
+    public void IsBoilerplateLine_IsCaseInsensitive()
+    {
+        // Act & Assert
+        Assert.IsTrue(FfmpegOutputFilter.IsBoilerplateLine("DURATION: 00:00:01.00"));
+        Assert.IsTrue(FfmpegOutputFilter.IsBoilerplateLine("duration: 00:00:01.00"));
+    }
+}