Add build and run scripts; drop the broken Avalonia.Diagnostics reference

A wrapper around `dotnet build` earns nothing, so build.ps1/build.sh are the
full local gate instead — tool restore, package restore, format check, build,
test — in the order that fails cheapest first, with a non-zero exit on failure.
run.ps1/run.sh default to Debug and pass extra arguments through to the app.
Both flavours ship because the Desktop head targets Windows, Linux and macOS.

Writing them immediately paid for itself: the very first run failed restore on
Avalonia.Diagnostics 12.1.1, which does not exist — the package stops at 11.3.x
because Avalonia 12 moved the inspector into a separate tool with its own
installation. The reference had survived because it sat behind
Condition="'$(Configuration)' == 'Debug'", and `dotnet restore` evaluates with
the default configuration while every build so far had passed -c Release. So
`dotnet build -c Release` worked and a bare `dotnet restore` did not.

Reference removed rather than replaced: AvaloniaUI.DiagnosticsSupport pulls in a
separately installed tool, which is not a dependency to add to a skeleton without
asking. README and CLAUDE.md no longer promise F12, and both traps are written
down where the next person will hit them.

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

src/AvParser.Desktop/AvParser.Desktop.csproj

@@ -28,8 +28,13 @@
     <PackageReference Include="Serilog.Extensions.Logging" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(Configuration)' == 'Debug'">
-    <!-- F12 opens DevTools, which is the fastest way to see :compact/:medium/:expanded toggle. -->
-    <PackageReference Include="Avalonia.Diagnostics" />
-  </ItemGroup>
+  <!--
+    No Avalonia.Diagnostics here: it stops at 11.3.x and has no Avalonia 12 build. Avalonia 12
+    moved the inspector out of the framework into a separate tool with its own installation
+    (AvaloniaUI.DiagnosticsSupport + `.WithDeveloperTools()`), so there is no built-in F12.
+
+    Note also that Condition="'$(Configuration)' == 'Debug'" on a PackageReference is a trap:
+    `dotnet restore` evaluates with the default configuration, so the package still has to
+    resolve even if nothing ever builds that configuration.
+  -->
 </Project>