build(steam): stage steam_api64.dll beside the exe — one file, not the whole SDK

The SDK zip is 24 MB of headers, examples and tools we never build. The runtime
dependency is one native library, so scripts/steam-sdk.sh extracts exactly
sdk/redistributable_bin/win64/steam_api64.dll into steam/ (gitignored, like ffmpeg/ —
the script is the audit trail) and writes steam_appid.txt = 480, Valve's Spacewar app,
so SteamAPI.Init() resolves an app id locally before LlamaCasty has one of its own.

Two csproj details that would have cost a debugging session:
- Link="steam_api64.dll" puts it BESIDE the exe; without Link, None items keep their
  relative path and it lands in steam/, where the loader does not look.
- ExcludeFromSingleFile, for the same reason ffmpeg needed it: a native DLL otherwise
  gets swallowed into the single-file bundle and cannot be loaded at runtime.

steam_appid.txt is Debug-only — Valve's guidance is not to ship it — and the publish
output confirms it: dll present at the root, no appid file, exe still 127 MB (proof the
DLL was not bundled).

Also queued, as asked: preview undo (Ctrl+Z / Ctrl+Shift+Z) in edit mode, noted as the
partner to the defaults-vs-current split — undo covers the slip, defaults cover the mess.

ytLive.csproj

@@ -100,6 +100,21 @@
     <Compile Remove="Controls\DeveloperPanel.xaml.cs" />
   </ItemGroup>
 
+  <!-- Steamworks runtime (scripts/steam-sdk.sh extracts it; steam/ is gitignored).
+       ⛔ Link puts it BESIDE the exe rather than under steam/, because the loader looks there.
+       ⛔ ExcludeFromSingleFile matters for the same reason as ffmpeg: a native DLL otherwise
+       gets swallowed into the single-file bundle and the app cannot load it at runtime. -->
+  <ItemGroup>
+    <None Remove="steam\**" />
+    <None Include="steam\steam_api64.dll" Link="steam_api64.dll"
+          CopyToOutputDirectory="PreserveNewest" CopyToPublishDirectory="Always"
+          ExcludeFromSingleFile="true" />
+    <!-- Local-only: lets SteamAPI.Init() resolve an app id outside the Steam client (480 =
+         Valve's Spacewar test app). Never published — Valve's guidance is to omit it. -->
+    <None Include="steam\steam_appid.txt" Link="steam_appid.txt" Condition="'$(Configuration)' == 'Debug'"
+          CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
   <ItemGroup>
     <None Include="THIRD-PARTY-NOTICES.txt" CopyToOutputDirectory="PreserveNewest" />
     <!-- The EULA and privacy policy travel with every copy: they are what the About screen and