Add position smoothing and movement snapping

Introduce simple position smoothing for normal proxy movement and snap special movements. Adds kProxyPositionLerpAlpha, Lerp/LerpPosition helpers, and ShouldSnapRemoteMovement to lerp proxy position on normal updates while snapping on cell_change, worldspace_change, and teleport. Update MoveProxyToRemotePlayer to use lerp or snap, keep heading snapping (TODO: wrapped angle smoothing), and add throttled/readable logs for smoothing start, first smoothed movement, snaps, and idle-on-cell-mismatch. Minor messaging change on disconnect to leave proxy idle at last position. Update docs (architecture, setup, protocol) and dev log to reflect the new behavior and testing notes; dynamic spawning/multiple proxies remain out of scope.

protocol/player-sync.md

@@ -72,12 +72,14 @@ player exists.
 - The fake client removes disconnected remote players from its state table.
 - The plugin moves the single placed test proxy actor from a thread-safe
   remote-player snapshot on the game-thread update path.
+- Normal proxy movement is smoothed with a simple position lerp, while
+  `cell_change`, `worldspace_change`, and `teleport` transforms snap directly to
+  avoid slow movement across large discontinuities.
 
 No Fallout 4 remote actor is dynamically spawned yet.
 
 ## Planned Later
 
-- Add interpolation or smoothing after snap movement is proven stable.
 - Add one proxy actor per remote player after the single-proxy test is stable.
 - Dynamically spawn remote actors only after placed-proxy movement is stable.