chore(motion): L.3 M6 — scrub stale ACDREAM_INTERP_MANAGER + dead fields

Cleans up dead code revealed by L.3 M2/M3:

GameWindow.cs:
- RemoteMotion.LastServerZ field deleted (only consumed by the M2-
  removed Step 5 landing fallback in TickAnimations; never read).
- RemoteMotion.TargetOrientation field deleted (audit § 1 flagged as
  DEAD; only ever written, never read).
- Stale ACDREAM_INTERP_MANAGER comments removed from RemoteMotion.Interp
  and OnLivePositionUpdated (the env-var no longer gates anything as
  of M2).
- Doc-comments on Interp + Position rewritten to describe the M2/M3
  production semantics (queue catch-up + REPLACE-style combiner).

CLAUDE.md:
- ACDREAM_INTERP_MANAGER env-var entry rewritten as a retirement note
  pointing at commit 40d88b9 (M2). The path it gated is now the
  default for player remotes.

Build green, dotnet test green (8 pre-existing failures unchanged on
this baseline).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -295,11 +295,6 @@ public sealed class GameWindow : IDisposable
         /// </summary>
         public double LastMoveToPacketTime;
         /// <summary>
-        /// Legacy field — no longer used for slerp (retail hard-snaps
-        /// per FUN_00514b90 set_frame). Kept to avoid churn.
-        /// </summary>
-        public System.Numerics.Quaternion TargetOrientation = System.Numerics.Quaternion.Identity;
-        /// <summary>
         /// Angular velocity seeded from UpdateMotion TurnCommand/TurnSpeed
         /// (π/2 × turnSpeed, signed). Applied per tick to body orientation
         /// via manual integration (bypassing <c>PhysicsBody.update_object</c>'s
@@ -334,34 +329,21 @@ public sealed class GameWindow : IDisposable
         /// <summary>
         /// Per-remote position-waypoint queue + catch-up math (retail's
         /// CPhysicsObj::InterpolateTo + InterpolationManager::adjust_offset).
-        /// Replaces the hard-snap-then-Euler-extrapolate path when
-        /// <c>ACDREAM_INTERP_MANAGER=1</c> — see Phase L.3.1 spec at
-        /// <c>docs/superpowers/specs/2026-05-02-l3-remote-entity-motion-design.md</c>.
-        /// Field exists from Task 3 onwards; consumed in Tasks 4 + 5.
+        /// Drives per-tick body translation for grounded player remotes
+        /// via <see cref="Position"/>.
         /// </summary>
         public AcDream.Core.Physics.InterpolationManager Interp { get; } =
             new AcDream.Core.Physics.InterpolationManager();
 
         /// <summary>
         /// Per-frame combiner for animation root motion + InterpolationManager
-        /// correction (Phase L.3.2). Consumed in TickAnimations to compute the
-        /// per-frame body.Position delta.
+        /// correction. Mirrors retail UpdatePositionInternal @ 0x00512c30:
+        /// queue catch-up REPLACES anim when active; anim stands when queue
+        /// is idle.
         /// </summary>
         public AcDream.Core.Physics.PositionManager Position { get; } =
             new AcDream.Core.Physics.PositionManager();
 
-        /// <summary>
-        /// Most recent server-broadcast Z coordinate from any UpdatePosition
-        /// (including mid-arc airborne UPs). Used by the
-        /// <c>ACDREAM_INTERP_MANAGER=1</c> per-tick path as a landing-fallback
-        /// floor: if gravity drags the body's Z below this value while
-        /// <see cref="Airborne"/> is still set, force-land locally because
-        /// the server has effectively told us where the ground is even if
-        /// it never sent an IsGrounded=true UP. Initialized to NaN so the
-        /// fallback is a no-op until the first UP arrives.
-        /// </summary>
-        public float LastServerZ = float.NaN;
-
         /// <summary>
         /// Diagnostic-only (gated on <c>ACDREAM_REMOTE_VEL_DIAG=1</c>): the
         /// previous UpdatePosition's world position + timestamp. The per-tick
@@ -3444,30 +3426,12 @@ public sealed class GameWindow : IDisposable
             // from ServerVelocity / ServerMoveTo which the legacy path
             // already handles correctly.
             //
-            // Was previously gated on ACDREAM_INTERP_MANAGER=1; the env-var
-            // path's per-tick TickAnimations counterpart is regressed
-            // (issue #40). M2 keeps the OnLivePositionUpdated half (which
-            // is correct) and rewrites the per-tick half — see TickAnimations.
             if (IsPlayerGuid(update.Guid))
             {
                 // Orientation always snaps on receipt — InterpolationManager walks
                 // position only; heading would otherwise lag the queue.
                 rmState.Body.Orientation = rot;
 
-                // Track the most recent GROUNDED server-broadcast Z. Read by
-                // the per-tick landing-fallback in TickAnimations: if gravity
-                // drags the body more than 0.5 m below this floor while still
-                // airborne, we force-land locally even when the server never
-                // sent an IsGrounded=true UP for the actual landing frame.
-                //
-                // Only updated for grounded UPs — mid-arc airborne UPs would
-                // raise this value to the player's peak Z, then the body's
-                // descent would cross (peak - 0.5) and trigger a force-land
-                // mid-air, producing the user-reported "small landing in the
-                // air before landing on the ground" when jumping while moving.
-                if (update.IsGrounded)
-                    rmState.LastServerZ = worldPos.Z;
-
                 // Diagnostic (ACDREAM_REMOTE_VEL_DIAG=1): roll the previous
                 // server-pos snapshot forward AND print the per-UP comparison
                 // between the max sequencer speed observed since last UP and
@@ -3643,7 +3607,6 @@ public sealed class GameWindow : IDisposable
             // a halved "observed" rate → visible slow-start. Formula-only
             // is stable and simple; hard-snap fixes any drift.
             rmState.Body.Orientation = rot;
-            rmState.TargetOrientation = rot;
             rmState.LastServerPos = worldPos;
             rmState.LastServerPosTime = nowSec;
             // Align the body's physics clock with our clock so update_object