refactor(streaming): capture landblock build origin

src/AcDream.App/Streaming/LandblockBuildRequest.cs

@@ -0,0 +1,36 @@
+namespace AcDream.App.Streaming;
+
+/// <summary>
+/// Immutable world-origin center captured by the update thread when a
+/// landblock load is admitted. Worker construction and render publication use
+/// this same value, so a later recenter cannot combine two coordinate frames.
+/// </summary>
+public readonly record struct LandblockBuildOrigin
+{
+    public LandblockBuildOrigin(int centerX, int centerY)
+    {
+        CenterX = centerX;
+        CenterY = centerY;
+        IsSpecified = true;
+    }
+
+    public int CenterX { get; }
+    public int CenterY { get; }
+
+    /// <summary>
+    /// Distinguishes a captured origin at the valid map coordinate (0,0)
+    /// from an old compatibility request that carries no origin at all.
+    /// </summary>
+    public bool IsSpecified { get; }
+}
+
+/// <summary>
+/// Complete immutable input to one CPU-side landblock build.
+/// <see cref="Generation"/> is carried for matching and diagnostics only;
+/// residency policy remains owned by <see cref="StreamingController"/>.
+/// </summary>
+public readonly record struct LandblockBuildRequest(
+    uint LandblockId,
+    LandblockStreamJobKind Kind,
+    ulong Generation,
+    LandblockBuildOrigin Origin);