feat(entitlement): IEntitlement seam — Steam DLC answers, everything fails closed

The first half of surge day 1. One question, "may this session emit Pro output?", with
one method on the interface so nothing downstream knows which rail proved it.

SteamEntitlement reads the lifetime SKU the way Valve intends: SteamAPI.Init() then
SteamApps.BIsDlcInstalled(dlcAppId). Four different failure paths all resolve to the
free tier rather than to Pro or to a crash — no DLC id configured, no Steam client,
steam_api64.dll missing (a throw on first touch), and simply not owned. A copied folder
run without Steam is the common case, not an exceptional one.

NotConfigured() is the shipping default until Steamworks creates the DLC app; it answers
false without ever touching the native library, so today's builds behave exactly as they
did before this commit.

HasProOutput is now IsPremium || DeveloperMode || rail, with an internal EntitlementOverride
test seam (internal, because a public setter here is a licence bypass). Four tests: no id
reads false, no client reads false without throwing, the fake reports what a test sets, and
the view model takes its answer from the rail.

Steamworks.NET 2024.8.0 added; the native DLL still comes from scripts/steam-sdk.sh, so the
csproj change is three lines and no formatting churn.

ViewModels/MainViewModel.OutputProfile.cs

@@ -1,4 +1,5 @@
 using ytLive.Helpers;
+using ytLive.Services.Entitlement;
 
 namespace ytLive.ViewModels;
 
@@ -23,7 +24,26 @@ public partial class MainViewModel
     /// settable inside a development build (see <see cref="DevBuild"/>), so this expression
     /// reduces to <c>IsPremium</c> in a Release binary.</para>
     /// </summary>
-    public bool HasProOutput => IsPremium || DeveloperMode;
+    public bool HasProOutput => IsPremium || DeveloperMode || _entitlement.IsPro;
+
+    /// <summary>
+    /// The entitlement rail for this session. Lifetime = Steam DLC; until the DLC app id exists it
+    /// is <see cref="SteamEntitlement.NotConfigured"/>, which reads false (fail closed). Tests
+    /// inject a fake through <see cref="EntitlementOverride"/>.
+    /// </summary>
+    private IEntitlement _entitlement = SteamEntitlement.NotConfigured();
+
+    /// <summary>Test seam: swap the rail without a Steam client. Internal — the test assembly is
+    /// the only caller, and a public setter here would be a licence bypass surface.</summary>
+    internal IEntitlement EntitlementOverride
+    {
+        get => _entitlement;
+        set
+        {
+            _entitlement = value;
+            OnPropertyChanged(nameof(HasProOutput));
+        }
+    }
 
     private bool _developerMode;