fix(physics): route remote Positions on contact, not walkability (AP-140)

The two gates that decide whether an accepted remote Position is interpolated
or hard-snapped read `Airborne`, which is `!Body.OnWalkable` — WALKABILITY.
Retail reads CONTACT: InterpolationManager::adjust_offset @0x00555D30 gates its
entire body on `transient_state & 1` @0x00555D52, so a retail body in contact
with a non-walkable face still interpolates.

The two predicates disagree in exactly one state — in contact, not on walkable
ground — which 204d0ae0 turned from unreachable into ordinary. Before it, the
per-tick forge made every non-airborne remote walkable by construction, so the
disagreement could not occur.

Both gates now read `!Body.InContact`: ApplyRemoteContactRouting's flight
carve-out and OnPosition's player-remote arm.

`Airborne` is deliberately NOT re-derived from CONTACT. That would perturb all
five of its writers and contradict a pinned assertion in
RemoteTeleportPlacementTests.Apply_PendingGroundToSteepContact_ (InContact:
true, OnWalkable: false -> Assert.True(remote.Airborne)); a previous
implementer attempted it and correctly backed out rather than editing the
assertion. This narrower shape touches no existing test.

AP-140's register row is retired in this commit, as the row itself specified.

Honest scope: this is a faithfulness fix, not a visible one. ACE derives its
IsGrounded flag with the same floor_z test, so during a slide it almost
certainly reports not-grounded, the classifier returns NoPositionOperation, and
neither arm is taken. Expect no observable change against ACE.

Suite 11,027 passed / 4 skipped / 0 failed (baseline 11,023).

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

tests/AcDream.App.Tests/Physics/RemotePlacementDriveFixture.cs

@@ -96,7 +96,21 @@ internal sealed class RemotePlacementDriveFixture : IDisposable
             Orientation = Quaternion.Identity,
             LastUpdateTime = 1d,
             State = PhysicsStateFlags.Gravity,
-            TransientState = TransientStateFlags.Active,
+            // AP-140 (retired 2026-08-04): the remote stands on the flat
+            // source landblock, so the sweep's SetPositionInternal commit
+            // would derive CONTACT (`contact_plane_valid` @0x00515430) and
+            // ON_WALKABLE (`contact_plane.N.z >= floor_z`
+            // @0x00515465-0x0051548E) for it. Both bits now have to be
+            // present, because the accepted-Position routing gates read
+            // retail's CONTACT predicate
+            // (`InterpolationManager::adjust_offset` @0x00555D52) rather than
+            // the client `Airborne` walkability flag: a body with a bare
+            // `Active` transient state is in FREE FLIGHT and every routing
+            // test would take the free-flight snap arm. Tests that want free
+            // flight clear these explicitly.
+            TransientState = TransientStateFlags.Active
+                | TransientStateFlags.Contact
+                | TransientStateFlags.OnWalkable,
         };
         body.SnapToCell(SourceCell, body.Position, body.Position);
         Lifetime.Entities.SetPhysicsBody(record, body);