fix #270: settle sweep at spawn placement - contact from the compressed first gravity frame

The [SPAWN-PLACE] probe showed placements succeeding with contact=False:
find_placement_pos validates the spot but the sphere sits a few cm above
the floor with no touch. Retail gains spawn contact from the FIRST
GRAVITY FRAME (every CPhysicsObj simulates, falls, touches); our
stationary remotes never run a physics frame. The seed now compresses
that settle: a short downward ResolveWithTransition from the server
position snaps the body onto the floor, and its touch grants the contact
plane + CONTACT/ON_WALKABLE via the verbatim commit. No floor within
reach = stays airborne, exactly like retail's fall.

The retry predicate now watches the CONTACT transient (the flag
contact_allows_move reads) instead of ContactPlaneValid - a DR-tick
writeback can set plane DATA from last-known state without real contact,
which is why the heavy attackers in the retry-session log got exactly
one placement attempt and then 250+ refused Falling dispatches.

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

src/AcDream.App/Physics/LiveEntityNetworkUpdateController.cs

@@ -178,43 +178,51 @@ internal sealed class LiveEntityNetworkUpdateController
             height = 1.835f;
         }
 
-        ResolveResult placement = _physicsEngine.ResolvePlacement(
+        var moverFlags = IsPlayerGuid(serverGuid)
+            ? AcDream.Core.Physics.ObjectInfoState.IsPlayer
+              | AcDream.Core.Physics.ObjectInfoState.EdgeSlide
+            : AcDream.Core.Physics.ObjectInfoState.EdgeSlide;
+
+        // Retail's spawn contact comes from the FIRST GRAVITY FRAME, not the
+        // placement itself: every retail CPhysicsObj simulates, so a freshly
+        // placed creature falls the few centimetres onto the floor and the
+        // transition's touch grants the contact plane. Our stationary remotes
+        // never run a physics frame (the DR tick resolves only movers), so
+        // the settle is compressed here: a short downward sweep from the
+        // server position. Its touch handler produces exactly the state
+        // retail's first frame would (position snapped onto the floor,
+        // contact plane + CONTACT/ON_WALKABLE committed below). A sweep that
+        // finds no floor (true airborne spawn) leaves the body airborne.
+        AcDream.Core.Physics.ResolveResult settle = _physicsEngine.ResolveWithTransition(
             worldPos,
+            worldPos + new System.Numerics.Vector3(0f, 0f, -0.5f),
             cellId,
-            radius,
-            height,
+            sphereRadius: radius,
+            sphereHeight: height,
             stepUpHeight: 0.4f,
             stepDownHeight: 0.4f,
-            moverFlags: IsPlayerGuid(serverGuid)
-                ? AcDream.Core.Physics.ObjectInfoState.IsPlayer
-                  | AcDream.Core.Physics.ObjectInfoState.EdgeSlide
-                : AcDream.Core.Physics.ObjectInfoState.EdgeSlide,
+            isOnGround: false,
+            body: remote.Body,
+            moverFlags: moverFlags,
             movingEntityId: entity.Id);
         if (AcDream.Core.Physics.PhysicsDiagnostics.DumpMotionEnabled)
         {
             Console.WriteLine(
-                $"[SPAWN-PLACE] guid={serverGuid:X8} cell=0x{cellId:X8} ok={placement.Ok} "
-                + $"contact={placement.InContact} walkable={placement.OnWalkable}");
+                $"[SPAWN-PLACE] guid={serverGuid:X8} cell=0x{cellId:X8} ok={settle.Ok} "
+                + $"contact={settle.InContact} walkable={settle.OnWalkable} "
+                + $"z={worldPos.Z:F3}->{settle.Position.Z:F3}");
         }
-        if (!placement.Ok)
-            return; // unplaceable — stays airborne, like a failed retail placement
+        if (!settle.Ok || !settle.InContact)
+            return; // no floor within reach — stays airborne like retail's fall
 
-        remote.Body.Position = placement.Position;
-        remote.Body.ContactPlaneValid = placement.InContact;
-        if (placement.InContact)
-        {
-            remote.Body.ContactPlane = placement.ContactPlane;
-            remote.Body.ContactPlaneCellId = placement.ContactPlaneCellId;
-            remote.Body.ContactPlaneIsWater = placement.ContactPlaneIsWater;
-            remote.Body.GroundNormal = placement.ContactPlane.Normal;
-        }
+        remote.Body.Position = settle.Position;
 
         AcDream.Core.Physics.PhysicsObjUpdate.CommitSetPositionTransition(
             remote.Body,
-            placement.InContact,
-            placement.OnWalkable,
-            placement.CollisionNormalValid,
-            placement.CollisionNormal,
+            settle.InContact,
+            settle.OnWalkable,
+            settle.CollisionNormalValid,
+            settle.CollisionNormal,
             previousContact: false,
             previousOnWalkable: false,
             remote.Movement.HitGround,
@@ -727,17 +735,19 @@ internal sealed class LiveEntityNetworkUpdateController
             remote.Body.Orientation = entity.Rotation;
             remote.Body.Position = entity.Position;
         }
-        // #270: run the retail spawn placement so the body has real ground
+        // #270: run the retail spawn settle so the body has real ground
         // contact BEFORE the funnel below dispatches this packet's actions —
         // an attack swing needs contact_allows_move true to animate. Retried
-        // (not creation-only) while the body has never been successfully
-        // placed: creation during the login flood can precede streaming
-        // residency or the entity's cell hydration, and retail's own answer
-        // to "object addressed before its cell exists" is the
-        // CObjectMaint lost-cell list — park it, re-place when the cell is
-        // available. The no-contact-and-no-plane predicate is exactly
-        // "never placed"; one success ends the retries.
-        if (!remote.Body.InContact && !remote.Body.ContactPlaneValid)
+        // (not creation-only) while the body lacks the CONTACT transient
+        // (the flag contact_allows_move reads — NOT ContactPlaneValid, which
+        // a DR writeback can set from last-known plane data without real
+        // contact): creation during the login flood can precede streaming
+        // residency or cell hydration, and retail's own answer to "object
+        // addressed before its cell exists" is the CObjectMaint lost-cell
+        // list — park it, re-place when the cell is available. One grounded
+        // settle ends the retries; a genuinely airborne remote (mid-jump
+        // player) fails the floor probe harmlessly until it lands.
+        if (!remote.Body.InContact)
         {
             SeedRemoteSpawnPlacement(
                 remote,