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>

MainPage.xaml.cs

@@ -222,19 +222,21 @@ public sealed partial class MainPage : Page
             throw new ArgumentException("Thumbnail interval must be at least one second.");
 
         EncoderMode encoder = Enum.Parse<EncoderMode>(SelectedEncoder());
-        return new(
-            _inputs.ToList(),
-            EndCardBox.Text.Trim(),
-            OutputFolderBox.Text.Trim(),
-            OutputNameBox.Text.Trim(),
-            FfmpegPathBox.Text.Trim(),
-            FfprobePathBox.Text.Trim(),
-            encoder,
-            Number(TrimBox.Text, "trim start"),
-            Number(FadeBox.Text, "fade"),
-            Number(HoldBox.Text, "end-card hold"),
-            interval,
-            MoveSourcesBox.IsChecked == true);
+        return new RenderSettings
+        {
+            InputFiles = _inputs.ToList(),
+            EndCardPath = EndCardBox.Text.Trim(),
+            OutputDirectory = OutputFolderBox.Text.Trim(),
+            OutputName = OutputNameBox.Text.Trim(),
+            FfmpegPath = FfmpegPathBox.Text.Trim(),
+            FfprobePath = FfprobePathBox.Text.Trim(),
+            Encoder = encoder,
+            TrimStart = Number(TrimBox.Text, "trim start"),
+            FadeSeconds = Number(FadeBox.Text, "fade"),
+            EndCardHoldSeconds = Number(HoldBox.Text, "end-card hold"),
+            ThumbnailInterval = interval,
+            MoveSourcesToOriginals = MoveSourcesBox.IsChecked == true,
+        };
     }
 
     private void ReplaceInputs(IReadOnlyList<string> paths)
@@ -325,19 +327,21 @@ public sealed partial class MainPage : Page
 
     private void SaveSettings()
     {
-        AppSettingsStore.Save(new AppSettings(
-            OutputFolderBox.Text.Trim(),
-            EndCardBox.Text.Trim(),
-            FfmpegPathBox.Text.Trim(),
-            FfprobePathBox.Text.Trim(),
-            PreviewPlayerPathBox.Text.Trim(),
-            SelectedTheme(),
-            SelectedEncoder(),
-            TrimBox.Text.Trim(),
-            FadeBox.Text.Trim(),
-            HoldBox.Text.Trim(),
-            IntervalBox.Text.Trim(),
-            MoveSourcesBox.IsChecked == true));
+        AppSettingsStore.Save(new AppSettings
+        {
+            OutputFolder = OutputFolderBox.Text.Trim(),
+            EndCardPath = EndCardBox.Text.Trim(),
+            FfmpegPath = FfmpegPathBox.Text.Trim(),
+            FfprobePath = FfprobePathBox.Text.Trim(),
+            PreviewPlayerPath = PreviewPlayerPathBox.Text.Trim(),
+            Theme = SelectedTheme(),
+            Encoder = SelectedEncoder(),
+            TrimStart = TrimBox.Text.Trim(),
+            FadeSeconds = FadeBox.Text.Trim(),
+            EndCardHoldSeconds = HoldBox.Text.Trim(),
+            ThumbnailInterval = IntervalBox.Text.Trim(),
+            MoveSourcesToOriginals = MoveSourcesBox.IsChecked == true,
+        });
     }
 
     private string SelectedTheme() => (ThemeBox.SelectedItem as ComboBoxItem)?.Tag?.ToString() ?? "Dark";