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>

src/AcDream.App/Streaming/LandblockStreamJob.cs

@@ -0,0 +1,28 @@
+using AcDream.Core.World;
+
+namespace AcDream.App.Streaming;
+
+/// <summary>
+/// A job posted to <see cref="LandblockStreamer"/>'s inbox. Either a load
+/// (fetch this landblock from the dats and build its CPU-side mesh data)
+/// or an unload (release any state tied to this landblock on the render
+/// thread's next Tick drain).
+/// </summary>
+public abstract record LandblockStreamJob(uint LandblockId)
+{
+    public sealed record Load(uint LandblockId) : LandblockStreamJob(LandblockId);
+    public sealed record Unload(uint LandblockId) : LandblockStreamJob(LandblockId);
+}
+
+/// <summary>
+/// Outbox record the render thread drains. Either a successful load, a
+/// failed load (logged and ignored until region recenters off/back), or
+/// an unload notification (tells the render thread to release GPU state
+/// for this landblock id).
+/// </summary>
+public abstract record LandblockStreamResult(uint LandblockId)
+{
+    public sealed record Loaded(uint LandblockId, LoadedLandblock Landblock) : LandblockStreamResult(LandblockId);
+    public sealed record Failed(uint LandblockId, string Error) : LandblockStreamResult(LandblockId);
+    public sealed record Unloaded(uint LandblockId) : LandblockStreamResult(LandblockId);
+}