fix(A.5 T4-T6): bootstrap guard + dead enum + test cleanups

Code review on commits 7bcabab/fb6b61e/326b698 flagged 2 Important +
4 Minor issues. Apply all fixes:

Important:
- Two-tier RecenterTo + MarkResidentFromBootstrap now throw
  InvalidOperationException on misuse — calling RecenterTo before the
  bootstrap silently emitted the entire window as fresh loads (no
  demotes/unloads since _tierResidence was empty), a correctness hazard
  that produced no exception. Calling MarkResidentFromBootstrap twice
  silently dropped accumulated tier state. Both now crash loudly via
  a _bootstrapped flag.
- Dropped TierResidence.None from the enum — never assigned, never
  checked; absence from the dictionary already encodes "not resident."

Minor:
- Renamed test: RecenterTo_FirstTick_* → ComputeFirstTickDiff_FirstTick_*
  (the test calls ComputeFirstTickDiff, not RecenterTo).
- Strengthened RecenterTo_PlayerWalks_NullToFar_* with assertions for
  ToPromote.Count==3 (the x=102 column promoting Far→Near) and
  ToUnload.Empty (everything within hysteresis).
- Replaced System.Math.Abs with Math.Abs in new code to match the
  file's existing `using System;` convention.

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

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

@@ -22,7 +22,7 @@ public class StreamingRegionTwoTierTests
     }
 
     [Fact]
-    public void RecenterTo_FirstTick_SplitsLoadIntoNearAndFar()
+    public void ComputeFirstTickDiff_FirstTick_SplitsLoadIntoNearAndFar()
     {
         // near=1, far=3 → near window is 3×3=9, far window is 7×7-3×3=40 LBs.
         var region = new StreamingRegion(centerX: 100, centerY: 100, nearRadius: 1, farRadius: 3);
@@ -52,6 +52,13 @@ public class StreamingRegionTwoTierTests
             Assert.Contains(id, diff.ToLoadFar);
         }
         Assert.Empty(diff.ToLoadNear);
+        // The 3 LBs at x=102, y in {99,100,101} were Far from old center
+        // (distance 2) and are now Near from new center (distance ≤1).
+        // They should land in ToPromote.
+        Assert.Equal(3, diff.ToPromote.Count);
+        // All resident LBs from the old window are within hysteresis of
+        // the new center (max distance 4 ≤ FarRadius+2=5), so nothing unloads.
+        Assert.Empty(diff.ToUnload);
     }
 
     [Fact]