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>

src/AcDream.App/Streaming/StreamingRegion.cs

@@ -26,6 +26,13 @@ public sealed class StreamingRegion
     // Two-tier residence tracking: maps each resident LB to its current tier.
     private readonly Dictionary<uint, TierResidence> _tierResidence = new();
 
+    // Set true after MarkResidentFromBootstrap. The two-tier RecenterTo
+    // requires this state to be seeded; calling RecenterTo before the
+    // bootstrap silently emits the entire window as fresh loads (no demotes,
+    // no unloads, since _tierResidence is empty), which is a correctness
+    // hazard. The flag converts that into a loud InvalidOperationException.
+    private bool _bootstrapped;
+
     /// <summary>
     /// 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
@@ -132,6 +139,12 @@ public sealed class StreamingRegion
     /// </summary>
     public void MarkResidentFromBootstrap()
     {
+        if (_bootstrapped)
+            throw new InvalidOperationException(
+                "MarkResidentFromBootstrap was already called; calling it again would " +
+                "reset accumulated tier-residence state and silently drop differential " +
+                "data built up by interim RecenterTo calls.");
+
         _tierResidence.Clear();
         for (int dx = -FarRadius; dx <= FarRadius; dx++)
         {
@@ -140,14 +153,15 @@ public sealed class StreamingRegion
                 int nx = CenterX + dx;
                 int ny = CenterY + dy;
                 if (nx < 0 || nx > 0xFF || ny < 0 || ny > 0xFF) continue;
-                int absDx = System.Math.Abs(dx);
-                int absDy = System.Math.Abs(dy);
+                int absDx = Math.Abs(dx);
+                int absDy = Math.Abs(dy);
                 var id = EncodeLandblockId(nx, ny);
                 _tierResidence[id] = (absDx <= NearRadius && absDy <= NearRadius)
                     ? TierResidence.Near
                     : TierResidence.Far;
             }
         }
+        _bootstrapped = true;
     }
 
     /// <summary>
@@ -165,6 +179,13 @@ public sealed class StreamingRegion
     /// </summary>
     public TwoTierDiff RecenterTo(int newCx, int newCy)
     {
+        if (!_bootstrapped)
+            throw new InvalidOperationException(
+                "Two-tier RecenterTo called before MarkResidentFromBootstrap. " +
+                "First call ComputeFirstTickDiff to enqueue the bootstrap loads, " +
+                "then MarkResidentFromBootstrap to seed _tierResidence, then RecenterTo " +
+                "for subsequent observer moves.");
+
         int nearUnloadThreshold = NearRadius + 2;
         int farUnloadThreshold  = FarRadius  + 2;
 
@@ -183,8 +204,8 @@ public sealed class StreamingRegion
                 int nx = newCx + dx;
                 int ny = newCy + dy;
                 if (nx < 0 || nx > 0xFF || ny < 0 || ny > 0xFF) continue;
-                int absDx = System.Math.Abs(dx);
-                int absDy = System.Math.Abs(dy);
+                int absDx = Math.Abs(dx);
+                int absDy = Math.Abs(dy);
                 bool inNear = absDx <= NearRadius && absDy <= NearRadius;
                 var id = EncodeLandblockId(nx, ny);
                 newCenterIds.Add(id);
@@ -213,9 +234,9 @@ public sealed class StreamingRegion
             var current = kvp.Value;
             int lbX     = (int)((id >> 24) & 0xFFu);
             int lbY     = (int)((id >> 16) & 0xFFu);
-            int absDx   = System.Math.Abs(lbX - newCx);
-            int absDy   = System.Math.Abs(lbY - newCy);
-            int distance = System.Math.Max(absDx, absDy);  // Chebyshev
+            int absDx   = Math.Abs(lbX - newCx);
+            int absDy   = Math.Abs(lbY - newCy);
+            int distance = Math.Max(absDx, absDy);  // Chebyshev
 
             if (newCenterIds.Contains(id))
             {
@@ -317,6 +338,8 @@ public readonly record struct RegionDiff(
     IReadOnlyList<uint> ToUnload);
 
 /// <summary>
-/// Tracks which tier a landblock currently occupies in the two-tier streaming model.
+/// Tracks which tier a landblock currently occupies in the two-tier streaming
+/// model. Absence from the dictionary encodes "not resident"; the enum has no
+/// None member to avoid suggesting a third runtime state.
 /// </summary>
-internal enum TierResidence { None, Far, Near }
+internal enum TierResidence { Far, Near }