[Event Leaderboard] Replace client-side zone calculation with server TierDelta

[yinnie]

Hiro 1.33 update adds TierDelta to IEventLeaderboardScore. This now exposes the server's computed tier outcome per player. Now it's possible to remove the client-side zone boundary logic. And to demonstrate how to use TierDelta directly.

Changes

• Previously, the client re-derived promotion and demotion cutoffs by reading ChangeZones percentages or RewardTiers rank ranges from the leaderboard config, then comparing each player's rank against those cutoffs.
• The new implementation reads TierDelta directly from each score and detects zone transitions.

Testing

• Tested locally against a local Nakama + Hiro server using the Food Battles event leaderboard with 4 accounts.
• For testing, the server config reward tier rank ranges were adjusted to fit a 4-player cohort (demotion from rank 3+)
• Rank 1: tier_change: +1 (promote)
• Rank 2: tier_change: 0 (stay)
• Ranks 3-4: tier_change: -1 (demote)
• and the reset schedule shortened to */3 * * * * with a 90-second duration to allow multiple phases in a single session.

Phase 1. Bronze tier 0.
All promoting.

Phase 2. Silver tier 1 (showing demotion, based on our rank config)

[CLAassistant]

All committers have signed the CLA.

[trandromeda]

So satisfying!

[trandromeda]

Thank you for the thorough description and testing results @yinnie. Great changes

[trandromeda]

Before deploying, we should update our Heroic Cloud instance first so the changes take place without breaking