[Fix] Sync Tablet::_cumulative_point with TabletMeta::_cumulative_layer_point

## Problem Description

There is a critical synchronization issue between the in-memory cumulative point
and its persistent storage:

- Tablet::_cumulative_point (in-memory, runtime value)
- TabletMeta::_cumulative_layer_point (persistent, stored in RocksDB)

The root cause is that these two values were never properly synchronized:

1. After compaction updates _cumulative_point, it was never written to _cumulative_layer_point
2. On BE restart, Tablet constructor hardcoded _cumulative_point to -1, ignoring the
   value loaded from RocksDB
3. This caused cumulative compaction to restart from scratch after every BE restart

## Data Flow Analysis

### BE Startup/Restart Flow:
```
DataDir::load()
  → TabletMetaManager::traverse_headers() [iterate RocksDB]
    → TabletManager::load_tablet_from_meta(meta_binary from RocksDB)
      → TabletMeta::deserialize(meta_binary)
        → TabletMeta::init_from_pb()
          → _cumulative_layer_point = tablet_meta_pb.cumulative_layer_point()
      → std::make_shared<Tablet>(_engine, tablet_meta, data_dir)
        → Tablet::Tablet() constructor
```

**Before this fix**: Constructor hardcoded _cumulative_point = -1, losing RocksDB value
**After this fix**: Constructor loads _cumulative_point from TabletMeta

### Compaction Update Flow:
```
CumulativeCompaction::execute_compact()
  → update_cumulative_point()
    → Tablet::set_cumulative_layer_point(new_point)  [updates _cumulative_point]
  → Tablet::save_meta()
```

**Before this fix**: save_meta() only saved TabletMeta without syncing _cumulative_point
**After this fix**: save_meta() syncs _cumulative_point to TabletMeta before persisting

## Solution

This commit adds bidirectional synchronization:

1. **Load path** (tablet.cpp:260):
   ```cpp
   _cumulative_point(_tablet_meta->cumulative_layer_point())
   ```
   Initialize from TabletMeta on construction (BE restart/clone/restore)

2. **Save path** (tablet.cpp:341):
   ```cpp
   _tablet_meta->set_cumulative_layer_point(_cumulative_point);
   ```
   Sync to TabletMeta before persisting to RocksDB

## Impact

### Fixed Scenarios:
- ✅ BE restart: Cumulative point persists across restarts
- ✅ Clone: Target replica inherits correct cumulative point
- ✅ Restore: Restored tablet keeps original cumulative point

### New Tablet Creation:
- Still correctly initializes to -1 (TabletMeta constructor sets it to -1)

## Related Issue

This also resolves the existing TODO comment:
```cpp
// TODO(ygl): lost some information here, such as cumulative layer point
// engine_storage_migration_task.cpp:348
```

Author: carolinchen-docker

be/src/olap/tablet.cpp

@@ -257,7 +257,7 @@ Tablet::Tablet(StorageEngine& engine, TabletMetaSharedPtr tablet_meta, DataDir*
           _last_cumu_compaction_success_millis(0),
           _last_base_compaction_success_millis(0),
           _last_full_compaction_success_millis(0),
-          _cumulative_point(K_INVALID_CUMULATIVE_POINT),
+          _cumulative_point(_tablet_meta->cumulative_layer_point()),
           _newly_created_rowset_num(0),
           _last_checkpoint_time(0),
           _cumulative_compaction_type(cumulative_compaction_type),
@@ -337,6 +337,8 @@ Status Tablet::init() {
 // if it's a primary replica
 void Tablet::save_meta() {
     check_table_size_correctness();
+    // Sync the in-memory cumulative point to tablet meta before saving
+    _tablet_meta->set_cumulative_layer_point(_cumulative_point);
     auto res = _tablet_meta->save_meta(_data_dir);
     CHECK_EQ(res, Status::OK()) << "fail to save tablet_meta. res=" << res
                                 << ", root=" << _data_dir->path();