feat(app): Phase A.1 — job + result records for LandblockStreamer

LandblockStreamJob (Load/Unload) and LandblockStreamResult
(Loaded/Failed/Unloaded) are the channel payload types the next
task's LandblockStreamer will use. Separate file because they're
shared between the worker thread and the render thread and deserve
a focused home.

Folds in two carryover nits from the Task 1 fix review:
- Stale "radius + 1" comments in StreamingRegionTests updated to
  match the real radius+2 threshold (no numeric-assertion changes).
- Single-step recenter test now asserts Visible.Count == 25 and
  Resident.Count == 30, locking in the Visible/Resident semantic
  split behaviorally.

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

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

@@ -45,17 +45,22 @@ public class StreamingRegionTests
     [Fact]
     public void RecenterTo_SingleStepEast_LoadsColumn_NoUnloadsDueToHysteresis()
     {
-        // Radius 2 → unload threshold is radius+1 = 3.
+        // Radius 2 → unload threshold is radius+2 = 4.
         // Starting center (50,50) covers X in [48..52]. Step to (51,50):
         // new coverage X in [49..53]. New column is x=53 (5 entries).
-        // Departing column would be x=48, but |48-51| = 3 which equals the
-        // threshold, so it stays loaded (hysteresis keeps radius+1).
+        // Departing column x=48 is at distance |48-51| = 3, which is within
+        // the radius+2 threshold, so it stays loaded (hysteresis keeps radius+2).
         var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
 
         var diff = region.RecenterTo(51, 50);
 
         Assert.Equal(5, diff.ToLoad.Count);
         Assert.Empty(diff.ToUnload);
+        // Visible is strictly the 5×5 window around (51, 50).
+        Assert.Equal(25, region.Visible.Count);
+        // Resident includes the hysteresis-retained x=48 column (5 entries)
+        // plus the full new window, for 30 total.
+        Assert.Equal(30, region.Resident.Count);
     }
 
     [Fact]
@@ -63,7 +68,7 @@ public class StreamingRegionTests
     {
         // Starting (50,50) covers X in [48..52]. Step to (53,50):
         // new coverage X in [51..55]. New columns: x=53,54,55 (15 entries).
-        // x=48 is now 5 away → unload. x=49,50 still within radius+1 → keep.
+        // x=48 is now 5 away, > radius+2 = 4 → unload. x=49 is 4 away, not > 4 → keep. x=50 is 3 away, not > 4 → keep.
         var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
 
         var diff = region.RecenterTo(53, 50);