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

This commit fixes the issue where the cumulative_layer_point in TabletMeta
was never synced with the in-memory _cumulative_point in Tablet, causing:

1. TabletMeta::_cumulative_layer_point was never updated after compaction
2. Tablet restart would lose the cumulative point progress

Changes:
- Tablet constructor now initializes _cumulative_point from TabletMeta
- Tablet::save_meta() now syncs _cumulative_point to TabletMeta before saving

Note: For snapshot/migration scenarios, the cumulative_layer_point will be
persisted when the new tablet meta is saved after copy.

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();