fix(app): Phase A.1 — encode landblock IDs with 0xFFFF terminator, not 0xFFFE

ROOT CAUSE of the "giant ball with spikes" terrain corruption that
the previous two hotfix attempts (lock + synchronous loading) failed
to address. Threading was a red herring all along.

AC dat conventions:
  0xAAAA0xFFFF — LandBlock dat (terrain heightmap)
  0xAAAA0xFFFE — LandBlockInfo dat (static-object metadata)

WorldView.NeighborLandblockIds correctly uses 0xFFFF. My
StreamingRegion.EncodeLandblockId from Phase A.1 Task 1 used 0xFFFE
by mistake. Every streaming load was therefore calling
LandblockLoader.Load with the LandBlockInfo id, which makes
DatCollection ask DatBinReader to read a LandBlock from the
LandBlockInfo file. The reader's internal buffer position lands in
the middle of the wrong file's bytes, ReadBytesInternal asks for an
out-of-range slice, throws ArgumentOutOfRangeException, and the
landblocks that DON'T throw return half-populated LandBlock objects
whose Height[] arrays contain garbage. Garbage Z values render as
the spike pattern.

The kicker: my Task-1 review fix added a test
(Constructor_SmallRadius_IDsMatchEncodingRule) that asserted
Assert.Contains(0x1234FFFEu, region.Visible). The test was passing
because it pinned the wrong value. I literally codified the bug.

Fix: change EncodeLandblockId's terminator from 0xFFFEu to 0xFFFFu
and update the test to assert 0x1234FFFFu. The XML doc on Visible
now explicitly explains the 0xFFFF/0xFFFE distinction so this can't
recur.

The previous two hotfixes (_datLock in c991fb2, synchronous streamer
in 531c9f9) stay in place — _datLock is defensive belt-and-suspenders
that documents which entry points read dats, and synchronous loading
is correct-by-default until we decide whether to reintroduce
background loading (Phase A.3 may make it unnecessary anyway).

212 tests green. With this fix the streaming should actually work.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Streaming/StreamingRegion.cs

@@ -21,10 +21,18 @@ public sealed class StreamingRegion
     private readonly HashSet<uint> _resident = new();
 
     /// <summary>
-    /// Landblock IDs (8.8 coordinate form: <c>(lbX &lt;&lt; 24) | (lbY &lt;&lt; 16) | 0xFFFE</c>)
-    /// in the current visible window. This is strictly the (2r+1)×(2r+1) set;
-    /// it does NOT include hysteresis-retained landblocks outside the window.
-    /// Use <see cref="Resident"/> to enumerate everything actually loaded.
+    /// Landblock IDs in the current visible window in the AC 8.8 coordinate
+    /// form: <c>(lbX &lt;&lt; 24) | (lbY &lt;&lt; 16) | 0xFFFF</c>. The trailing
+    /// <c>0xFFFF</c> selects the LandBlock dat (terrain heightmap); the
+    /// matching LandBlockInfo (static-object metadata) is at <c>0xFFFE</c>
+    /// on the same coordinates and is resolved internally by
+    /// <c>LandblockLoader</c>.
+    ///
+    /// <para>
+    /// This set is strictly the (2r+1)×(2r+1) window; it does NOT include
+    /// hysteresis-retained landblocks outside the window. Use
+    /// <see cref="Resident"/> to enumerate everything actually loaded.
+    /// </para>
     /// </summary>
     public IReadOnlyCollection<uint> Visible => _visible;
 
@@ -61,8 +69,17 @@ public sealed class StreamingRegion
         _resident.UnionWith(_visible);
     }
 
+    /// <summary>
+    /// Encode a landblock at (lbX, lbY) into the AC dat id form. Always uses
+    /// the <c>0xFFFF</c> terminator (LandBlock = terrain). The earlier
+    /// version of this method used <c>0xFFFE</c> by mistake — that's the
+    /// LandBlockInfo id, and asking <c>LandblockLoader.Load</c> to read a
+    /// LandBlock at the LandBlockInfo coords corrupts the dat reader's
+    /// buffer position, returning a half-populated <c>LandBlock.Height[]</c>
+    /// array which renders as wildly distorted "ball with spikes" terrain.
+    /// </summary>
     internal static uint EncodeLandblockId(int lbX, int lbY)
-        => ((uint)lbX << 24) | ((uint)lbY << 16) | 0xFFFEu;
+        => ((uint)lbX << 24) | ((uint)lbY << 16) | 0xFFFFu;
 
     /// <summary>
     /// Recompute the visible window around a new center and return the

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

@@ -103,11 +103,17 @@ public class StreamingRegionTests
     [Fact]
     public void Constructor_SmallRadius_IDsMatchEncodingRule()
     {
-        // Verify EncodeLandblockId is correct (not swapped shifts).
-        // radius=0 at (0x12, 0x34) → exactly one entry, id = 0x1234FFFE.
+        // Verify EncodeLandblockId emits the LandBlock terminator (0xFFFF),
+        // not the LandBlockInfo terminator (0xFFFE). The earlier version of
+        // this test pinned 0xFFFE and codified the bug that produced "ball
+        // of spikes" terrain in Phase A.1's first live run — LandblockLoader
+        // was being asked to read the LandBlockInfo file as a LandBlock,
+        // which corrupted the dat reader's buffer position and returned a
+        // half-populated heightmap. radius=0 at (0x12, 0x34) → exactly one
+        // entry, id = 0x1234FFFF.
         var region = new StreamingRegion(cx: 0x12, cy: 0x34, radius: 0);
 
         Assert.Single(region.Visible);
-        Assert.Contains(0x1234FFFEu, region.Visible);
+        Assert.Contains(0x1234FFFFu, region.Visible);
     }
 }