fix(streaming): rebuild the streaming window on a far login-spawn — the cold-spawn "hole"

Root cause (confirmed live via the landblock-load probe): the streaming window
marks every landblock "resident" at bootstrap (MarkResidentFromBootstrap) BEFORE
their async loads land. When a character spawns far from the startup center
(0xA9B4 Holtburg), the login-spawn recenter runs RecenterTo against that stale,
half-loaded window — and RecenterTo trusts _tierResidence, so it never
re-enqueues the old/new window overlap. The overlap band is marked resident but
its loads never completed, leaving a permanent HOLE of landblocks that are never
requested (zero BUILD-NULL — they're simply skipped). The player spawns in the
hole: its landblock never loads, so terrain/NPCs/player never draw — the
"world not loading / invisible / scenery behind" symptom.

Probe evidence (spawn at 0xADAF): the player's column 0xAD loads Y=0xA3-0xAA and
0xB0-0xC0 but is MISSING Y=0xAB-0xAF — and the player is at 0xADAF (Y=0xAF), dead
in the gap. 0xADAFFFFF: never [lb] ADD'd, never BUILD-NULL'd.

Fix: a far login-spawn moves the render origin exactly like an outdoor teleport
(which already calls StreamingController.ForceReloadWindow to drop the stale
window + re-bootstrap fresh around the new origin — GameWindow.cs ~5725). The
login-spawn recenter now flags the same rebuild; because the spawn handler runs
on the network thread and ForceReloadWindow is render-thread-only, the flag is
set there and consumed in OnUpdateFrame's streaming block before the Tick.

Verified live: 0xADAFFFFF now `[lb] ADD entities=216`, full world draws
(flatCount=12807), and the player transitions PENDING(hidden) -> DRAWSET PRESENT
(composes with the RelocateEntity pending-recovery fix, 315af02f). No-op for a
normal Holtburg login (spawn == startup center → guard false). Full suite 4007
green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -880,6 +880,17 @@ public sealed class GameWindow : IDisposable
     private AcDream.Core.Net.WorldSession? _liveSession;
     private int _liveCenterX;
     private int _liveCenterY;
+    // Set by the login-spawn recenter (network thread) when the spawn landblock
+    // differs from the startup streaming center; consumed on the render thread
+    // in OnUpdateFrame to trigger StreamingController.ForceReloadWindow(). A far
+    // login-spawn moves the render origin just like an outdoor teleport, so the
+    // streaming region — bootstrapped around the stale startup center — must be
+    // rebuilt fresh around the spawn. Without this, RecenterTo treats the
+    // old/new window overlap as already-resident (MarkResidentFromBootstrap marks
+    // residence before the async loads land), leaving a permanent hole of
+    // resident-but-never-loaded landblocks where the player spawns — the
+    // cold-spawn "invisible player / world not loading" bug (2026-07-03).
+    private volatile bool _pendingForceReloadWindow;
     private uint _liveEntityIdCounter = 1_000_000u;  // well above any dat-hydrated id
 
     // K-fix1 (2026-04-26): cached at startup so per-frame branches are
@@ -3197,6 +3208,13 @@ public sealed class GameWindow : IDisposable
                 $"to ({lbX},{lbY}) for player spawn @0x{p.LandblockId:X8}");
             _liveCenterX = lbX;
             _liveCenterY = lbY;
+            // The origin jumped — the streaming region is still bootstrapped
+            // around the stale startup center with residence marked before its
+            // async loads landed. Flag a full window rebuild (consumed on the
+            // render thread in OnUpdateFrame) so the region re-bootstraps fresh
+            // around the spawn, re-loading the landblocks RecenterTo would
+            // otherwise skip as already-resident — the cold-spawn streaming hole.
+            _pendingForceReloadWindow = true;
         }
 
         // #135: the instant we know the player spawned into a SEALED dungeon,
@@ -7826,6 +7844,17 @@ public sealed class GameWindow : IDisposable
                 observerCx = (int)((cellLb >> 8) & 0xFFu);
                 observerCy = (int)(cellLb & 0xFFu);
             }
+            // Consume the login-spawn far-recenter flag (network thread → render
+            // thread): drop the stale startup window + null the region so this
+            // Tick re-bootstraps the whole window fresh around the spawn origin.
+            // Same mechanism the outdoor-teleport path uses (line ~5725). Fixes
+            // the cold-spawn streaming hole (resident-but-never-loaded landblocks
+            // the RecenterTo diff skipped as already-resident).
+            if (_pendingForceReloadWindow)
+            {
+                _pendingForceReloadWindow = false;
+                _streamingController.ForceReloadWindow();
+            }
             _streamingController.Tick(observerCx, observerCy, insideDungeon);
 
             // Re-inject persistent entities rescued from unloaded landblocks