perf(content): add typed prepared asset source

Introduce explicit loaded, missing, and corrupt outcomes over typed pak keys, validate package/DAT identity at open, add zero-I/O TOC probes, preserve cancellation, and route the package path through RuntimeOptions. Production injection follows in the next checkpoint.

tests/AcDream.App.Tests/RuntimeOptionsTests.cs

@@ -24,6 +24,9 @@ public sealed class RuntimeOptionsTests
         var opts = RuntimeOptions.Parse(AnyDatDir, EmptyEnv());
 
         Assert.Equal(AnyDatDir, opts.DatDir);
+        Assert.Equal(
+            Path.Combine(AnyDatDir, "acdream.pak"),
+            opts.PreparedAssetPath);
         Assert.False(opts.LiveMode);
         Assert.Equal("127.0.0.1", opts.LiveHost);
         Assert.Equal(9000, opts.LivePort);
@@ -49,6 +52,19 @@ public sealed class RuntimeOptionsTests
         Assert.False(opts.HasLiveCredentials);
     }
 
+    [Fact]
+    public void PreparedAssetPath_DefaultsBesideDats_AndAllowsOneOverride()
+    {
+        Assert.Equal(
+            Path.Combine(AnyDatDir, "acdream.pak"),
+            RuntimeOptions.Parse(AnyDatDir, EmptyEnv()).PreparedAssetPath);
+
+        var overridden = RuntimeOptions.Parse(
+            AnyDatDir,
+            Env(new() { ["ACDREAM_PAK_PATH"] = "D:/prepared/acdream.pak" }));
+        Assert.Equal("D:/prepared/acdream.pak", overridden.PreparedAssetPath);
+    }
+
     [Fact]
     public void LiveMode_Set_ExactlyByValue1()
     {