fix(streaming): close #54 — plumb JobKind through BuildLandblockForStreaming

Bug A's fix (commit `9217fd9`) patched at the worker output by stripping
entities from far-tier `LoadedLandblock`s after the full `LoadNear` path
ran. The worker still wasted CPU on `LandBlockInfo` reads + entity
hydration + `SceneryGenerator` math + interior-cell walks for ~544
far-tier LBs at radius=12, just to throw the work away.

This commit plumbs `LandblockStreamJobKind` through to the factory so the
worker can branch at the source:

- `LandblockStreamer.cs`: replace the `Func<uint, LoadedLandblock?>`
  factory with `Func<uint, LandblockStreamJobKind, LoadedLandblock?>` as
  the primary ctor signature. Add a back-compat overload that wraps the
  old single-arg signature (`(id, _) => loadLandblock(id)`) so existing
  test code keeps compiling without modification — the 5 ctor sites in
  `LandblockStreamerTests.cs` now resolve to the overload. `HandleJob`
  passes `load.Kind` to the factory; the post-load entity-strip is
  retained as a `Debug.Assert` + Release safety net.

- `GameWindow.cs`: `BuildLandblockForStreaming(uint, JobKind)` branches
  on `kind == LoadFar` at the top — reads only the `LandBlock` heightmap
  dat and returns a `LoadedLandblock` with `Array.Empty<WorldEntity>()`.
  Skips `LandblockLoader.Load` (which reads `LandBlockInfo`),
  `BuildSceneryEntitiesForStreaming`, and `BuildInteriorEntitiesForStreaming`
  entirely. Near-tier path is unchanged. Both call sites updated to pass
  the kind through the lambda: `(id, kind) => BuildLandblockForStreaming(id, kind)`.

Tests: 1688/1696 (8 pre-existing physics/input failures unchanged).
Streaming-targeted filter (30 tests covering LandblockStreamer +
StreamingController + StreamingRegion) all green via the back-compat
overload — no test code needed updating.

Per-LB worker cost on far-tier: was ~tens of ms (full hydration,
including LandBlockInfo + scenery generation + interior cells); now a
single `LandBlock` dat read (~sub-ms).

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -1652,7 +1652,7 @@ public sealed class GameWindow : IDisposable
         // it can call LandblockMesh.Build without a dat read — _heightTable and
         // _blendCtx are read-only after init, _surfaceCache is ConcurrentDictionary (T9).
         _streamer = new AcDream.App.Streaming.LandblockStreamer(
-            loadLandblock: id => BuildLandblockForStreaming(id),
+            loadLandblock: (id, kind) => BuildLandblockForStreaming(id, kind),
             buildMeshOrNull: (id, lb) =>
             {
                 if (lb is null || _heightTable is null || _blendCtx is null || _surfaceCache is null)
@@ -4639,8 +4639,18 @@ public sealed class GameWindow : IDisposable
     /// DatReaderWriter) and pure CPU work. No GL calls here.
     ///
     /// MVP scope: stabs only. Scenery + interior added in Task 8.
+    ///
+    /// ISSUE #54 (post-A.5): far-tier loads (<c>kind == LoadFar</c>) skip
+    /// LandBlockInfo + scenery + interior hydration. They return only the
+    /// LandBlock heightmap dat record + an empty entity list — enough for
+    /// terrain-mesh build on the next phase. Near-tier loads run the full
+    /// path. This replaces Bug A's post-load entity strip in
+    /// <see cref="AcDream.App.Streaming.LandblockStreamer"/> with an
+    /// early-out at the source.
     /// </summary>
-    private AcDream.Core.World.LoadedLandblock? BuildLandblockForStreaming(uint landblockId)
+    private AcDream.Core.World.LoadedLandblock? BuildLandblockForStreaming(
+        uint landblockId,
+        AcDream.App.Streaming.LandblockStreamJobKind kind)
     {
         if (_dats is null) return null;
 
@@ -4653,14 +4663,31 @@ public sealed class GameWindow : IDisposable
         // contention by pre-building render-thread work on the worker.
         lock (_datLock)
         {
-            return BuildLandblockForStreamingLocked(landblockId);
+            return BuildLandblockForStreamingLocked(landblockId, kind);
         }
     }
 
-    private AcDream.Core.World.LoadedLandblock? BuildLandblockForStreamingLocked(uint landblockId)
+    private AcDream.Core.World.LoadedLandblock? BuildLandblockForStreamingLocked(
+        uint landblockId,
+        AcDream.App.Streaming.LandblockStreamJobKind kind)
     {
         if (_dats is null) return null;
 
+        // ISSUE #54: far-tier early-out — heightmap only, empty entities.
+        // Skips the LandBlockInfo dat read AND all entity hydration (stabs
+        // + buildings) AND the SceneryGenerator AND interior cells. Cuts
+        // worker-thread cost per far-tier LB from ~tens of ms to a single
+        // dat read.
+        if (kind == AcDream.App.Streaming.LandblockStreamJobKind.LoadFar)
+        {
+            var heightmapOnly = _dats.Get<DatReaderWriter.DBObjs.LandBlock>(landblockId);
+            if (heightmapOnly is null) return null;
+            return new AcDream.Core.World.LoadedLandblock(
+                landblockId,
+                heightmapOnly,
+                System.Array.Empty<AcDream.Core.World.WorldEntity>());
+        }
+
         var baseLoaded = AcDream.Core.World.LandblockLoader.Load(_dats, landblockId);
         if (baseLoaded is null) return null;
 
@@ -8157,7 +8184,7 @@ public sealed class GameWindow : IDisposable
             _streamer.Dispose();
 
             _streamer = new AcDream.App.Streaming.LandblockStreamer(
-                loadLandblock: id => BuildLandblockForStreaming(id),
+                loadLandblock: (id, kind) => BuildLandblockForStreaming(id, kind),
                 buildMeshOrNull: (id, lb) =>
                 {
                     if (lb is null || _heightTable is null || _blendCtx is null || _surfaceCache is null)