feat(A.5 T13): StreamingController two-tier Tick

Replaces the single-radius Tick with a two-tier model that consumes
StreamingRegion's TwoTierDiff (5-list) and routes to the appropriate
JobKind:

- ToLoadFar    -> _enqueueLoad(id, LoadFar)
- ToLoadNear   -> _enqueueLoad(id, LoadNear)
- ToPromote    -> _enqueueLoad(id, PromoteToNear)
- ToDemote     -> _state.RemoveEntitiesFromLandblock(id) on render thread
- ToUnload     -> _enqueueUnload(id)

Drain switch handles Loaded (terrain + entity layer), Promoted (entity
layer only -- terrain already loaded), Unloaded, Failed, WorkerCrashed.

Constructor signature: nearRadius/farRadius separate ints. Old single-
radius ctor removed; existing single-radius tests updated to pass
nearRadius=farRadius for backward-compat coverage.

GameWindow's enqueueLoad lambda updated from (id =>...) to (id, kind) =>
to match new Action<uint, LandblockStreamJobKind> signature; radius: arg
renamed to nearRadius:/farRadius: (both set to _streamingRadius until T16
wires the full two-tier env-var parsing).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Streaming/StreamingControllerTwoTierTests.cs

@@ -0,0 +1,38 @@
+using System.Collections.Generic;
+using AcDream.App.Streaming;
+using AcDream.Core.Terrain;
+using AcDream.Core.World;
+using Xunit;
+
+namespace AcDream.Core.Tests.Streaming;
+
+public class StreamingControllerTwoTierTests
+{
+    [Fact]
+    public void Tick_FirstCall_EnqueuesNearAndFarLoadsByTier()
+    {
+        var loads = new List<(uint Id, LandblockStreamJobKind Kind)>();
+        var unloads = new List<uint>();
+        var state = new GpuWorldState();
+
+        var ctrl = new StreamingController(
+            enqueueLoad: (id, kind) => loads.Add((id, kind)),
+            enqueueUnload: unloads.Add,
+            drainCompletions: _ => System.Array.Empty<LandblockStreamResult>(),
+            applyTerrain: (_, _) => { },
+            state: state,
+            nearRadius: 1,
+            farRadius: 3);
+
+        ctrl.Tick(observerCx: 100, observerCy: 100);
+
+        int nearCount = 0, farCount = 0;
+        foreach (var (_, kind) in loads)
+        {
+            if (kind == LandblockStreamJobKind.LoadNear) nearCount++;
+            else if (kind == LandblockStreamJobKind.LoadFar) farCount++;
+        }
+        Assert.Equal(9, nearCount);   // 3x3 inner ring (radius=1)
+        Assert.Equal(40, farCount);   // 7x7 - 3x3 outer ring (radius=3)
+    }
+}