refactor: replace PushbackManager scheduled task with event-driven CircularBoundingBox

[Copilot]

Replace the polling-based runAtFixedRate pushback system with a PlayerMoveEvent-driven approach using per-executor circular bounding boxes.

Changes

• CircularBoundingBox — new utility class for 2D circular containment checks using squared distance (no sqrt)
• PushbackManager — remove scheduled task, maintain a Map<UUID, CircularBoundingBox> per executor instead of a Set<UUID>
  • updateExecutorPosition(player) — moves the box center on executor move
  • checkPushback(player) — non-executors check isInside() against all active boxes, with 1s cooldown
• PlayerMoveListener — drives both methods on every position change
• PaperMain — remove PushbackManager.startTask() call

How it works

// Executor moves → box follows
PushbackManager.updateExecutorPosition(player)  // O(1) map lookup + center update

// Any player moves → check all executor boxes
PushbackManager.checkPushback(player)  // early-out on permission/cooldown, then isInside() per box

Both paths bail out fast for the common case (non-executor with no nearby boxes). The isInside check is a simple dx² + dz² ≤ r² comparison rather than getNearbyPlayers() entity scanning every second.

Warning

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