Remove legacy WPF app, flatten WinUI project to repo root, rename namespace

The repo carried two parallel UIs (WPF + WinUI) sharing Models/Services via
cross-directory Link includes. Now that WinUI is the only frontend, collapse
the structure so the WinUI project IS the repo root instead of a nested
sibling folder:

- Delete the WPF project entirely (App.xaml, MainWindow.xaml, csproj) and its
  bin/obj output
- Move AmiReel.WinUI/* up to the repo root (App, MainWindow, MainPage,
  DialogHelper, Assets, Package.appxmanifest, app.manifest, Properties,
  .github/instructions, AGENTS.md) via git mv, preserving history
- Rename AmiReel.WinUI.csproj -> AmiReel.csproj; regenerate the solution as
  AmiReel.slnx (the newer XML solution format) with a single project
- Rename namespace AmigaDB.VideoRenderer.{Models,Services} -> AmiReel.{...}
  and AmiReel_WinUI -> AmiReel across all files, including the embedded
  ffmpeg/ffprobe resource logical names in the csproj and ToolExtractor
- Models/ and Services/ no longer need the Link-based cross-directory
  <Compile Include>; they're picked up by the SDK's default globbing now
  that they live under the project directory
- Rename assets/ -> branding/ (source icon art) to avoid a case-insensitive
  collision with Assets/ (packaged tile art) once both sit at repo root
- Merge the two .gitignore files into one; track the PublishProfiles pubxml
  files instead of ignoring them (no secrets, and they keep publish
  reproducible across machines) as branding, gitignore, etc.
- Simplify publish-win-x64.ps1 (drop the -Target Wpf/WinUI switch, there's
  only one target now) and rewrite README.md to describe the single-project
  layout, build/run/publish commands, and file structure

Verified: dotnet build succeeds for both AmiReel.csproj and AmiReel.slnx, and
the built exe launches and renders identically to before the move.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

branding/README.txt

@@ -0,0 +1,22 @@
+AmiReel Icon Pack
+=================
+
+Contents:
+
+- AmiReel.ico
+  Windows multi-resolution application icon containing:
+  16, 24, 32, 48, 64, 128 and 256 px.
+
+- AmiReel-master-1254.png
+  Original transparent master artwork.
+
+- png/
+  Individual transparent PNG files:
+  16, 24, 32, 48, 64, 128, 256, 512 and 1024 px.
+
+Referenced from the project file as:
+
+  <ApplicationIcon>branding\AmiReel.ico</ApplicationIcon>
+
+Product: AmiReel — Amiga Video Renderer
+Brand: AmigaDB