Scene strip: move top row over the preview column and scale it; remove CRT bezel art and on-air seed

- PreviewPane.xaml: drop the OBS-style border Rectangle/Ellipse over webcam elements
- SceneThumbnailStrip: Viewbox scales the row; height follows width so it fills the preview column; staged+live tile has no border
- MainWindow.xaml: strip moved to column 1 only, leaving the layers cell empty; Margin 8,0 matches the preview
- MainViewModel.Background: stop seeding the CRT scene tiles and the on-air image
- ytLive.csproj: per-build counter (BuildCounter.txt, gitignored) replaces the commit-count stamp

Build stamp now increments on every build.

ytLive.csproj

@@ -40,7 +40,7 @@
            Baseline 241 = count at introduction, first display 13. Missing git or unparsed output
            falls back to the start (build never breaks). GUID + compile time stay in startup.log,
            where build-identity forensics actually live. -->
-      <BuildNumberBaseline>241</BuildNumberBaseline>
+      <BuildNumberBaseline>17</BuildNumberBaseline>
       <BuildNumberStart>13</BuildNumberStart>
     </PropertyGroup>
     <Exec Command="&quot;C:\Program Files\Git\cmd\git.exe&quot; rev-list --count HEAD"
@@ -49,7 +49,13 @@
     </Exec>
     <PropertyGroup>
       <CommitCount>$(CommitCountRaw.Trim())</CommitCount>
-      <BuildNumber>$([MSBuild]::Add($(BuildNumberStart), $([MSBuild]::Subtract($(CommitCount), $(BuildNumberBaseline)))))</BuildNumber>
+      <BuildCounterFile>$(ProjectDir)BuildCounter.txt</BuildCounterFile>
+      <PriorBuildCount Condition="Exists('$(BuildCounterFile)')">$([System.IO.File]::ReadAllText('$(BuildCounterFile)').Trim())</PriorBuildCount>
+      <PriorBuildCount Condition="'$(PriorBuildCount)' == ''">423</PriorBuildCount>
+      <BuildNumber>$([MSBuild]::Add($(PriorBuildCount), 1))</BuildNumber>
+    </PropertyGroup>
+    <WriteLinesToFile File="$(BuildCounterFile)" Lines="$(BuildNumber)" Overwrite="true" WriteOnlyWhenDifferent="false" />
+    <PropertyGroup>
       <BuildNumber Condition="! $([System.Text.RegularExpressions.Regex]::IsMatch('$(BuildNumber)', '^[0-9]+$'))">$(BuildNumberStart)</BuildNumber>
     </PropertyGroup>
     <ItemGroup>