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>

Models/RenderSettings.cs

@@ -7,23 +7,33 @@ public enum EncoderMode
     CpuX264
 }
 
-public sealed record RenderSettings(
-    IReadOnlyList<string> InputFiles,
-    string EndCardPath,
-    string OutputDirectory,
-    string OutputName,
-    string FfmpegPath,
-    string FfprobePath,
-    EncoderMode Encoder,
-    double TrimStart,
-    double FadeSeconds,
-    double EndCardHoldSeconds,
-    int ThumbnailInterval,
-    bool MoveSourcesToOriginals = true,
-    int Width = 3840,
-    int Height = 2160,
-    int FramesPerSecond = 50,
-    int ThumbnailWidth = 1280,
-    int ThumbnailHeight = 720);
+/// <summary>
+/// One render job's parameters. Uses named init properties rather than a positional
+/// constructor: TrimStart/FadeSeconds/EndCardHoldSeconds are three consecutive
+/// <see langword="double"/> members (and Width/Height/FramesPerSecond/ThumbnailWidth/
+/// ThumbnailHeight five consecutive <see langword="int"/> members), so a positional
+/// record would let arguments be silently transposed at a call site without the
+/// compiler catching it.
+/// </summary>
+public sealed record RenderSettings
+{
+    public required IReadOnlyList<string> InputFiles { get; init; }
+    public required string EndCardPath { get; init; }
+    public required string OutputDirectory { get; init; }
+    public required string OutputName { get; init; }
+    public required string FfmpegPath { get; init; }
+    public required string FfprobePath { get; init; }
+    public required EncoderMode Encoder { get; init; }
+    public required double TrimStart { get; init; }
+    public required double FadeSeconds { get; init; }
+    public required double EndCardHoldSeconds { get; init; }
+    public required int ThumbnailInterval { get; init; }
+    public bool MoveSourcesToOriginals { get; init; } = true;
+    public int Width { get; init; } = 3840;
+    public int Height { get; init; } = 2160;
+    public int FramesPerSecond { get; init; } = 50;
+    public int ThumbnailWidth { get; init; } = 1280;
+    public int ThumbnailHeight { get; init; } = 720;
+}
 
 public sealed record RenderProgress(double Percent, string Stage, string Message);