fix(net): timeslice WorldSession.Tick inbound drain (#2 flood)

After a teleport ACE floods a town's CreateObjects; WorldSession.Tick drained the
ENTIRE inbound queue every frame, each spawn hydrating mesh+textures under _datLock
on the render thread — monopolizing the update loop for ~a minute and starving the
streaming apply, so only the destination landblock loaded and neighbors trickled in
(visible at high render-FPS because update/render are separate Silk.NET callbacks).

Bound the per-frame drain to a ~4ms wall-clock budget once InWorld (handshake uses
the blocking PumpOnce path, never Tick). A time budget self-adapts to the highly
variable per-datagram cost; the tail stays queued (unbounded channel, FIFO) and
drains over the next few frames. Acks are queued per packet BEFORE the heavy handler
(WorldSession.cs:680), so deferring the tail only delays the tail's acks a few frames
— within ACE's tolerance (holtburger defers acks on a flush cadence; verified in
references/holtburger session/send.rs). Budget decision extracted as a pure testable
static (4 unit tests).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core.Net/WorldSession.cs

@@ -1,4 +1,5 @@
 using System.Buffers.Binary;
+using System.Diagnostics;
 using System.Net;
 using System.Threading.Channels;
 using AcDream.Core.Combat;
@@ -590,22 +591,50 @@ public sealed class WorldSession : IDisposable
         _netThread.Start();
     }
 
+    // Per-frame inbound time budget (#2 flood timeslice). On a teleport arrival ACE floods
+    // a town's CreateObjects; draining them ALL in one Tick (each hydrates mesh + textures
+    // under _datLock on the render thread) monopolized the update loop for ~a minute and
+    // starved the streaming apply, so neighbor landblocks trickled in. A wall-clock budget
+    // self-adapts to the highly variable per-datagram cost (cheap UpdatePosition vs an
+    // expensive clothed CreateObject); the tail drains over the next few frames, FIFO
+    // preserved, nothing lost (the inbox channel is unbounded). ~4ms leaves the rest of a
+    // 60fps update frame for streaming + physics.
+    private static readonly long InboundBudgetTicks = Stopwatch.Frequency / 1000 * 4;
+
     /// <summary>
-    /// Non-blocking pump. Drains any datagrams buffered by the background
-    /// net thread (Phase A.3), decodes them, and fires events. Call once
-    /// per game-loop frame. Returns the number of datagrams processed.
+    /// Non-blocking pump. Drains datagrams buffered by the background net thread (Phase A.3),
+    /// decodes them, and fires events. Call once per game-loop frame. Once in-world the drain
+    /// is bounded to ~4ms so a CreateObject flood can't monopolize the frame. Returns the
+    /// number of datagrams processed.
     /// </summary>
     public int Tick()
     {
         int processed = 0;
+        long start = Stopwatch.GetTimestamp();
         while (_inboundQueue.Reader.TryRead(out var bytes))
         {
             ProcessDatagram(bytes);
             processed++;
+            // Bound ONLY in-world: the handshake uses the blocking PumpOnce path, never Tick
+            // (the net thread that feeds _inboundQueue starts at Transition(State.InWorld)).
+            // Acks are queued per packet inside ProcessDatagram BEFORE the heavy handler, so
+            // deferring the tail only delays the tail's acks a few frames — within ACE's
+            // tolerance (holtburger defers acks on a flush cadence). The tail stays queued
+            // (unbounded channel, FIFO) and drains next frame.
+            if (InboundBudgetExceeded(CurrentState, start, Stopwatch.GetTimestamp(), InboundBudgetTicks))
+                break;
         }
         return processed;
     }
 
+    /// <summary>
+    /// Pure, testable decision for the per-frame inbound bound: stop draining only when
+    /// in-world AND the elapsed Stopwatch ticks have reached the budget. Extracted so the
+    /// gate logic is unit-tested without the ISAAC/decode/channel machinery.
+    /// </summary>
+    internal static bool InboundBudgetExceeded(State state, long startTicks, long nowTicks, long budgetTicks)
+        => state == State.InWorld && nowTicks - startTicks >= budgetTicks;
+
     /// <summary>
     /// Phase A.3: background receive loop. Runs on a dedicated daemon
     /// thread started at the end of <see cref="EnterWorld"/>. Continuously