refactor: update ec_volume.go to use ECContext

- Add ECContext field to EcVolume struct
- Initialize ECContext with default configuration in NewEcVolume()
- Update LocateEcShardNeedleInterval() to use ECContext.DataShards
- Phase 1: Always uses default 10+4 configuration
- No behavior change

weed/storage/erasure_coding/ec_volume.go

@@ -42,6 +42,7 @@ type EcVolume struct {
 	diskType                  types.DiskType
 	datFileSize               int64
 	ExpireAtSec               uint64    //ec volume destroy time, calculated from the ec volume was created
+	ECContext                 *ECContext // EC encoding parameters
 }
 
 func NewEcVolume(diskType types.DiskType, dir string, dirIdx string, collection string, vid needle.VolumeId) (ev *EcVolume, err error) {
@@ -78,6 +79,9 @@ func NewEcVolume(diskType types.DiskType, dir string, dirIdx string, collection
 		volume_info.SaveVolumeInfo(dataBaseFileName+".vif", &volume_server_pb.VolumeInfo{Version: uint32(ev.Version)})
 	}
 
+	// Initialize EC context with default configuration (Phase 1: always 10+4)
+	ev.ECContext = NewDefaultECContext(collection, vid)
+
 	ev.ShardLocations = make(map[ShardId][]pb.ServerAddress)
 
 	return
@@ -260,7 +264,7 @@ func (ev *EcVolume) LocateEcShardNeedleInterval(version needle.Version, offset i
 	if ev.datFileSize > 0 {
 		// To get the correct LargeBlockRowsCount
 		// use datFileSize to calculate the shardSize to match the EC encoding logic.
-		shardSize = ev.datFileSize / DataShardsCount
+		shardSize = ev.datFileSize / int64(ev.ECContext.DataShards)
 	}
 	// calculate the locations in the ec shards
 	intervals = LocateData(ErasureCodingLargeBlockSize, ErasureCodingSmallBlockSize, shardSize, offset, types.Size(needle.GetActualSize(size, version)))