feat(gobplustree): 添加页面结构定义与反序列化逻辑

新增了 B+ 树中页面的基本结构体定义，包括页头、单元格等。同时实现了
页头和整个页面数据的反序列化方法，用于从字节数据中解析出页面对象。
该功能为后续存储引擎的读取和管理页面提供基础支持。

page.go

@@ -0,0 +1,99 @@
+package gobplustree
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+type PageType int8
+type Offset uint32
+
+const (
+	PageTypeLeaf PageType = iota
+	PageTypeInternal
+)
+
+const (
+	CellHeaderSize = 17
+)
+
+type PageHead struct {
+	PageId          int64
+	Type            PageType
+	CellCount       int
+	FirstFreeOffset Offset
+}
+type Cell struct {
+	IsFree         bool
+	NextFreeOffset Offset
+	ActualSize     int
+	Start          Offset
+}
+
+type Page struct {
+	*PageHead
+	Cells []*Cell
+	data  []byte
+}
+
+func DeSerializePageHead(data []byte) (*PageHead, error) {
+	PageId := binary.LittleEndian.Uint64(data[0:8])
+	Type := PageType(data[8])
+	CellCount := int(data[9])
+	FirstFreeOffset := Offset(data[10])
+	return &PageHead{
+		PageId:          int64(PageId),
+		Type:            Type,
+		CellCount:       CellCount,
+		FirstFreeOffset: FirstFreeOffset,
+	}, nil
+}
+func DeSerializePage(data []byte) (*Page, error) {
+	head, err := DeSerializePageHead(data[0:BlockFileHeaderSize])
+	if err != nil {
+		return nil, err
+	}
+	if head.CellCount == 0 {
+		return &Page{
+			PageHead: head,
+			Cells:    []*Cell{},
+			data:     data,
+		}, nil
+	}
+	cells := make([]*Cell, head.CellCount)
+	// find cell offset array start
+	cellPOffset := 0
+	for i := 17; i < BlockFilePageSize; i++ {
+		t := binary.LittleEndian.Uint32(data[i : i+4])
+		if t != 0 {
+			cellPOffset = i
+			break
+		}
+	}
+
+	if cellPOffset == 0 {
+		return nil, errors.New("invalid page")
+	}
+	currentFreeOffset := head.FirstFreeOffset
+	for i := 0; i < head.CellCount; i++ {
+		cellOffset := binary.LittleEndian.Uint32(data[cellPOffset+i*4 : cellPOffset+i*4+4])
+		nextOffSet := binary.LittleEndian.Uint32(data[cellOffset : cellOffset+4])
+		cellSize := nextOffSet - cellOffset
+		cells[i] = &Cell{
+			IsFree:         false,
+			NextFreeOffset: 0,
+			ActualSize:     int(cellSize),
+			Start:          Offset(cellOffset),
+		}
+		if cellOffset == uint32(currentFreeOffset) {
+			cells[i].IsFree = true
+			cells[i].NextFreeOffset = Offset(binary.LittleEndian.Uint32(data[cellOffset+4 : cellOffset+8]))
+			currentFreeOffset = cells[i].NextFreeOffset
+		}
+	}
+	return &Page{
+		PageHead: head,
+		Cells:    cells,
+		data:     data,
+	}, nil
+}