[Bug] Region Balance Not Optimal - Some Regions Overloaded

## Description
Region balance algorithm is not optimal. Some regions become overloaded while others remain underutilized. Load not evenly distributed across stores, causing performance bottlenecks.

## Steps to Reproduce
1. PD manages 10 TiKV stores
2. One region has all hotspot traffic
3. Balance algorithm doesn't detect/redistribute
4. Store A has 90% resource usage, Store B has 20%
5. Overall cluster capacity underutilized

## Environment Information
- System: Region balancing
- Algorithm: Current balance logic
- Metrics: Latency, CPU, memory
- Optimization: Suboptimal

## Expected Behavior
- Even load distribution across stores
- Hotspot detection and mitigation
- Leader distribution balanced
- Peak resource usage <70%

## Actual Behavior
- Uneven load distribution
- Hotspots not detected
- Overloaded stores
- Underutilized capacity

## Code Reference
- File: `server/coordinator.go`
- Method: `BalanceRegionLeader()` - suboptimal algorithm
- Missing: Hotspot detection
- Missing: Advanced balancing strategies

## Additional Context
Level 2 optimization issue. Fix requires:
- Improving balance algorithm
- Adding hotspot detection
- Implementing adaptive load balancing
- Performance testing and tuning

## Suggested Labels
`performance`, `optimization`, `balancing`, `clustering`, `resource-management`