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/StreamingControllerTests.cs

@@ -14,7 +14,7 @@ public class StreamingControllerTests
         public List<uint> Unloads { get; } = new();
         public Queue<LandblockStreamResult> Pending { get; } = new();
 
-        public void EnqueueLoad(uint id) => Loads.Add(id);
+        public void EnqueueLoad(uint id, LandblockStreamJobKind _) => Loads.Add(id);
         public void EnqueueUnload(uint id) => Unloads.Add(id);
         public IReadOnlyList<LandblockStreamResult> DrainCompletions(int max)
         {
@@ -36,12 +36,13 @@ public class StreamingControllerTests
             drainCompletions: fake.DrainCompletions,
             applyTerrain: (_, _) => { },
             state: state,
-            radius: 2);
+            nearRadius: 2,
+            farRadius: 2);
 
         // Center at (50, 50); no landblocks loaded yet.
         controller.Tick(observerCx: 50, observerCy: 50);
 
-        // 5×5 window = 25 loads enqueued, 0 unloads.
+        // 5×5 window = 25 loads enqueued (nearRadius==farRadius so all go to ToLoadNear), 0 unloads.
         Assert.Equal(25, fake.Loads.Count);
         Assert.Empty(fake.Unloads);
     }
@@ -53,7 +54,7 @@ public class StreamingControllerTests
         var fake = new FakeStreamer();
         var controller = new StreamingController(
             fake.EnqueueLoad, fake.EnqueueUnload, fake.DrainCompletions,
-            (_, _) => { }, state, radius: 2);
+            (_, _) => { }, state, nearRadius: 2, farRadius: 2);
 
         controller.Tick(50, 50);
         fake.Loads.Clear();
@@ -72,7 +73,7 @@ public class StreamingControllerTests
         var applied = new List<LoadedLandblock>();
         var controller = new StreamingController(
             fake.EnqueueLoad, fake.EnqueueUnload, fake.DrainCompletions,
-            (lb, _) => applied.Add(lb), state, radius: 2);
+            (lb, _) => applied.Add(lb), state, nearRadius: 2, farRadius: 2);
 
         // Note: LoadedLandblock's actual fields are LandblockId, Heightmap,
         // Entities (positional record). Adjust if the first positional arg
@@ -99,7 +100,7 @@ public class StreamingControllerTests
         var fake = new FakeStreamer();
         var controller = new StreamingController(
             fake.EnqueueLoad, fake.EnqueueUnload, fake.DrainCompletions,
-            (_, _) => { }, state, radius: 2);
+            (_, _) => { }, state, nearRadius: 2, farRadius: 2);
 
         var lb = new LoadedLandblock(0x32320FFEu, new LandBlock(), System.Array.Empty<WorldEntity>());
         state.AddLandblock(lb);