feat(teleport A): StreamingController priority-apply for the teleport destination LB

Adds PriorityLandblockId (uint, default 0) + _deferredApply buffer.
DrainAndApply now: (1) applies up to budget from the deferred buffer,
(2) when PriorityLandblockId != 0, hunts the worker outbox in chunks
applying the priority LB immediately on match and buffering any
non-priority items drained past it for later frames,
(3) falls back to normal drain when no priority is set or not found.

Extracts ApplyResult(result) + ResultLandblockId(result) helpers so
both the priority and normal paths share identical side-effects.
No existing behaviour changes on the non-priority path.

21 streaming tests pass (19 existing + 2 new priority-apply tests).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,85 @@
+using System;
+using System.Collections.Generic;
+using AcDream.App.Streaming;
+using AcDream.Core.Terrain;
+using AcDream.Core.World;
+using DatReaderWriter.DBObjs;
+using Xunit;
+
+namespace AcDream.Core.Tests.Streaming;
+
+public class StreamingControllerPriorityApplyTests
+{
+    private static LandblockStreamResult.Loaded LoadedOf(uint canonicalId)
+        => new(canonicalId, LandblockStreamTier.Near,
+               new LoadedLandblock(canonicalId, new LandBlock(), Array.Empty<WorldEntity>()),
+               new LandblockMeshData(Array.Empty<TerrainVertex>(), Array.Empty<uint>()));
+
+    [Fact]
+    public void PriorityLandblock_isApplied_evenWhenBeyondPerFrameCap()
+    {
+        uint priority = StreamingRegion.EncodeLandblockIdForTest(169, 180);
+        var outbox = new Queue<LandblockStreamResult>(new LandblockStreamResult[]
+        {
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 0)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 1)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 2)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 3)),
+            LoadedOf(priority),
+        });
+
+        var applied = new List<uint>();
+        var state = new GpuWorldState();
+        var ctrl = new StreamingController(
+            enqueueLoad: (_, _) => { },
+            enqueueUnload: _ => { },
+            drainCompletions: max =>
+            {
+                var batch = new List<LandblockStreamResult>();
+                while (batch.Count < max && outbox.Count > 0) batch.Add(outbox.Dequeue());
+                return batch;
+            },
+            applyTerrain: (lb, _) => applied.Add(lb.LandblockId),
+            state: state, nearRadius: 4, farRadius: 12)
+        { MaxCompletionsPerFrame = 4 };
+
+        ctrl.PriorityLandblockId = priority;
+        ctrl.Tick(169, 180);
+
+        Assert.Contains(priority, applied);   // priority applied THIS tick
+        Assert.True(applied.Count <= 5);       // did not blindly flush the whole outbox
+    }
+
+    [Fact]
+    public void Deferred_nonPriority_completions_applyOnLaterFrames_withoutLoss()
+    {
+        // After the priority is found, the non-priority items drained past it must
+        // still be applied (over subsequent ticks), never dropped.
+        uint priority = StreamingRegion.EncodeLandblockIdForTest(169, 180);
+        var outbox = new Queue<LandblockStreamResult>(new LandblockStreamResult[]
+        {
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 0)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 1)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 2)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 3)),
+            LoadedOf(StreamingRegion.EncodeLandblockIdForTest(0, 4)),
+            LoadedOf(priority),
+        });
+        var applied = new List<uint>();
+        var state = new GpuWorldState();
+        var ctrl = new StreamingController(
+            (_, _) => { }, _ => { },
+            max => { var b = new List<LandblockStreamResult>(); while (b.Count < max && outbox.Count > 0) b.Add(outbox.Dequeue()); return b; },
+            (lb, _) => applied.Add(lb.LandblockId),
+            state, 4, 12) { MaxCompletionsPerFrame = 4 };
+        ctrl.PriorityLandblockId = priority;
+
+        ctrl.Tick(169, 180);  // priority + some others
+        ctrl.PriorityLandblockId = 0u;
+        ctrl.Tick(169, 180);  // drains the deferred remainder
+        ctrl.Tick(169, 180);
+
+        Assert.Contains(priority, applied);
+        Assert.Equal(6, applied.Count);  // all six applied, none lost
+    }
+}