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>

Services/FfmpegOutputFilter.cs

@@ -0,0 +1,40 @@
+namespace AmiReel.Services;
+
+/// <summary>
+/// Recognizes the FFmpeg/FFprobe startup banner and stream-info boilerplate that both the
+/// live log filter (<see cref="ProcessRunner"/>) and the error summarizer
+/// (<see cref="UserFacingErrors"/>) want to hide — the version/library banner, input/output
+/// stream dumps, and the final size/duration summary line are never useful to a user.
+/// </summary>
+public static class FfmpegOutputFilter
+{
+    private static readonly string[] BoilerplatePrefixes =
+    [
+        "ffmpeg version ",
+        "built with ",
+        "configuration:",
+        "libavutil",
+        "libavcodec",
+        "libavformat",
+        "libavdevice",
+        "libavfilter",
+        "libswscale",
+        "libswresample",
+        "Input #",
+        "Output #",
+        "Stream mapping:",
+        "Stream #",
+        "Metadata:",
+        "Duration:",
+        "Press [q] to stop",
+        "video:",
+        "audio:",
+        "subtitle:",
+        "other streams:",
+        "global headers:",
+        "muxing overhead:",
+    ];
+
+    public static bool IsBoilerplateLine(string line) =>
+        BoilerplatePrefixes.Any(prefix => line.StartsWith(prefix, StringComparison.OrdinalIgnoreCase));
+}