feat(motion): per-frame Interp.AdjustOffset in remote tick (L.3.1 Task 5)

Wraps the existing legacy per-frame remote tick (apply_current_movement
+ force-OnWalkable + Euler-extrapolate) in ACDREAM_INTERP_MANAGER=1
env-var guard. When set:
- if Interp.IsActive: rm.Body.Position += Interp.AdjustOffset(dt, pos, maxSpeed)
- still call body.UpdatePhysicsInternal so airborne arcs (gravity)
  continue to integrate via the OnLiveVectorUpdated-set velocity.

When env-var unset (default), legacy path runs unchanged.

Mirrors retail's per-tick CPhysicsObj::UpdateObjectInternal (acclient
@ 0x00513730) which calls InterpolationManager::adjust_offset
(@ 0x00555D30) every frame.

Old legacy path will be removed in Task 8 cleanup commit after visual
verification.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -5763,6 +5763,47 @@ public sealed class GameWindow : IDisposable
                 && serverGuid != _playerServerGuid
                 && _remoteDeadReckon.TryGetValue(serverGuid, out var rm))
             {
+                if (System.Environment.GetEnvironmentVariable("ACDREAM_INTERP_MANAGER") == "1")
+                {
+                    // ── NEW PATH: queued position-chase via InterpolationManager ──
+                    // (L.3.1 Task 5 — ACDREAM_INTERP_MANAGER=1 gates this path)
+                    //
+                    // Walking remotes have m_velocityVector == 0 in retail; all
+                    // visible horizontal motion comes from
+                    // InterpolationManager::adjust_offset (acclient @ 0x00555D30)
+                    // walking the body toward the head of the waypoint queue at
+                    // 2 × motion_max_speed × dt (clamped, 7.5 m/s fallback).
+                    //
+                    // Mirrors retail CPhysicsObj::UpdateObjectInternal
+                    // (acclient @ 0x00513730) which calls adjust_offset every frame
+                    // before UpdatePhysicsInternal integrates gravity.
+                    //
+                    // For airborne remotes, OnLiveVectorUpdated has set
+                    // body.Velocity (launch arc); we still call
+                    // UpdatePhysicsInternal below so gravity applies each frame and
+                    // produces the parabolic arc. The IsActive gate prevents
+                    // AdjustOffset from pulling against an in-flight arc when no
+                    // waypoints are queued for a jumping remote.
+                    if (rm.Interp.IsActive)
+                    {
+                        float maxSpeed = rm.Motion.GetMaxSpeed();
+                        System.Numerics.Vector3 delta = rm.Interp.AdjustOffset((double)dt, rm.Body.Position, maxSpeed);
+                        rm.Body.Position += delta;
+                    }
+
+                    // Gravity integration: retail's UpdatePhysicsInternal still
+                    // fires every frame regardless of the interpolation path.
+                    // For grounded remotes body.Velocity == 0 so this is a no-op;
+                    // for airborne remotes it applies gravity to the arc.
+                    rm.Body.UpdatePhysicsInternal(dt);
+
+                    ae.Entity.Position = rm.Body.Position;
+                    ae.Entity.Rotation = rm.Body.Orientation;
+                }
+                else
+                {
+                    // ── LEGACY PATH (UNCHANGED — kept until Task 8 cleanup) ──
+                    //
                     // Stop detection is handled explicitly on packet receipt:
                     //   - UpdateMotion with ForwardCommand flag CLEARED → Ready.
                     //   - UpdatePosition with HasVelocity flag CLEARED → StopCompletely.
@@ -6078,6 +6119,7 @@ public sealed class GameWindow : IDisposable
                     ae.Entity.Position = rm.Body.Position;
                     ae.Entity.Rotation = rm.Body.Orientation;
                 }
+            }
 
             // ── Get per-part (origin, orientation) from either sequencer or legacy ──
             IReadOnlyList<AcDream.Core.Physics.PartTransform>? seqFrames = null;