fix: lock ball to player sprite during possession to prevent visual trailing

When a player possesses the ball, the server sets ball velocity to 0 and
places it at an offset from the player. On the client, the player sprite
is predicted ahead via dead reckoning, but the ball was lerping slowly
toward the stale server position — causing it to visibly trail behind.

Now computes the server-side offset (ball pos - player pos) and applies
it to the local predicted sprite position, keeping the ball locked to
the player visually.

Author: tim4724

client/src/scenes/MultiplayerScene.ts

@@ -577,13 +577,36 @@ export class MultiplayerScene extends BaseGameScene {
       if (this.ball.x == null || this.ball.y == null || isNaN(this.ball.x) || isNaN(this.ball.y)) {
         this.ball.x = serverBall.x
         this.ball.y = serverBall.y
+      } else if (serverBall.possessedBy) {
+        // Ball is possessed — lock it to the possessing player's SPRITE position
+        // instead of lerping toward the server ball position.  The server places
+        // the ball at player + direction * POSSESSION_BALL_OFFSET, but the player
+        // sprite is predicted ahead of the server (dead reckoning / client prediction).
+        // Lerping would make the ball visibly trail the player.
+        const possessorSprite = this.players.get(serverBall.possessedBy)
+        if (possessorSprite) {
+          // Use the server ball position relative to the server player position
+          // to compute the offset, then apply it to the local sprite position.
+          const possessorState = state.players?.get(serverBall.possessedBy)
+          if (possessorState) {
+            const offsetX = serverBall.x - possessorState.x
+            const offsetY = serverBall.y - possessorState.y
+            this.ball.x = possessorSprite.x + offsetX
+            this.ball.y = possessorSprite.y + offsetY
+          } else {
+            this.ball.x = serverBall.x
+            this.ball.y = serverBall.y
+          }
+        } else {
+          this.ball.x = serverBall.x
+          this.ball.y = serverBall.y
+        }
       } else {
-        // Dead reckoning: extrapolate ball using last known velocity to fill gaps between patches.
-        // Skip when ball is possessed — velocity is stale and the ball tracks the player instead.
+        // Ball is free — dead reckoning extrapolation + lerp
         let targetX = this.lastBallServerX
         let targetY = this.lastBallServerY
 
-        if (!serverBall.possessedBy && this.lastBallStateReceivedAt > 0) {
+        if (this.lastBallStateReceivedAt > 0) {
           const dtS = Math.min((now - this.lastBallStateReceivedAt) / 1000, 0.1) // cap at 100ms
           // Integrate ball friction: 0.98 per 60Hz physics step
           const frictionScale = Math.pow(GAME_CONFIG.BALL_FRICTION, dtS * 60)