Hold proxy in hidden position when no remote

Add a temporary in-cell holding fallback for the single placed proxy actor when no valid same-cell remote player is available. Introduces kProxyHiddenHoldingPosition, ProxyLifecycleState, RemotePlayerSelection, MoveProxyToHoldingPosition and RestoreProxyForRemotePlayer, plus selection/state-tracking and throttled logs. The controller now moves the proxy to the hidden position on disconnect, disappearance, or cell-mismatch and snaps it back when a valid same-cell remote appears. Documentation and dev log updated to describe the lifecycle and rationale (docs/architecture.md, docs/dev-log.md, protocol/player-sync.md).

protocol/player-sync.md

@@ -27,7 +27,10 @@ lifecycle.
 The plugin's game-thread proxy actor controller can now read a copied snapshot
 of this remote-player state and move the single placed test proxy actor
 `F4TProxyRemotePlayer01REF` in `F4TTestCell01`. The networking thread still does
-not touch Fallout 4 actors or references.
+not touch Fallout 4 actors or references. If no valid same-cell remote player is
+available, the controller moves the placed proxy to a hidden holding position
+inside the same test cell instead of leaving it frozen at the last represented
+position.
 
 ## Current Remote Player State Model
 
@@ -89,6 +92,10 @@ player exists.
 - Normal proxy movement is smoothed toward the latest received target, while
   `cell_change`, `worldspace_change`, and `teleport` transforms snap directly to
   avoid slow movement across large discontinuities.
+- The game-thread proxy controller holds the single proxy at a hidden in-cell
+  position when the represented player disconnects, disappears, or leaves the
+  local test cell, then snaps back to a valid same-cell remote player before
+  resuming smoothing.
 - Transform packets now include basic movement state data for future animation
   work, but the proxy actor does not apply animations from that state yet.