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>

CLAUDE.md

@@ -552,17 +552,14 @@ via `PlayerMovementController.ApplyServerRunRate`) or from
   diagnostics (`[UM_RAW]`, `[SCFAST]`, `[SCFULL]`, `[SETCYCLE]`,
   `[FWD_WIRE]`, `[OMEGA_DIAG]`, `[SEQSTATE]`, `[PARTSDIAG]`,
   `[VEL_DIAG]`, `[UPCYCLE]`). Heavy.
-- ⚠️ `ACDREAM_INTERP_MANAGER=1` — **DO NOT ENABLE.** This was an
-  experimental rewrite (e94e791) of the per-tick remote motion path.
-  It's regressed: the env-var path drops the per-tick collision sweep
-  (`ResolveWithTransition`) that the default path retains, causing a
-  visible "staircase" pattern when remotes run up/down slopes (body
-  Z stays flat between UPs, snaps at each one) plus position blips
-  during steady-state motion. Default (env-var unset) uses the
-  working retail-port chain. The PositionManager class itself is
-  fine and retail-faithful; only the integration into per-tick was
-  wrong. To be re-done in a future L.3 follow-up phase as additive
-  refinement on top of the working chain.
+- *(retired 2026-05-05 by L.3 M2/M3)* `ACDREAM_INTERP_MANAGER` was an
+  env-var gate on an experimental per-tick remote motion path. L.3 M2
+  (commit 40d88b9) replaced both gates (`OnLivePositionUpdated` +
+  `TickAnimations`) with `IsPlayerGuid(...)` so player remotes use the
+  retail-faithful queue routing (InterpolationManager queue catch-up +
+  PositionManager combiner) unconditionally. NPCs and airborne player
+  remotes still flow through the legacy `apply_current_movement` +
+  `ResolveWithTransition` path. The env-var no longer toggles anything.
 
 ### Outbound motion wire format (acdream → ACE)