feat: v0.2.0 AriaEngine 自研存储引擎

- 新增 AriaEngine: LSM-Tree 页面式存储引擎，19 个模块，~3500 行 TS
  - page/: Slotted Page 格式 (header/slot/tuple/format) + CRC32
  - buffer/: Buffer Pool (LRU 缓存 + 驱逐策略)
  - index/: LSM-Tree (MemTable 红黑树 + SSTable + Bloom Filter + Merge Iterator)
  - wal/: WAL 日志 (二进制格式) + Checkpoint 管理
  - transaction/: MVCC 版本链 + 快照隔离
  - store/: IndexedDB / Memory 双后端抽象
  - compression/: LZ4 页面压缩

- 完整持久化: Schema 自动保存、SSTable 元数据管理、WAL 恢复
- 事务感知 CRUD: insert/update/delete 在事务中缓冲到 snapshot
- mode: 'aria' 激活自研引擎

- 新增 7 个测试文件，测试数 318 → 524，套件 20 → 27
  - aria-page.test.ts (32 tests): Page 格式单元测试
  - aria-index.test.ts (26 tests): Bloom Filter + MemTable
  - aria-sstable.test.ts (9 tests): SSTable Builder + Reader
  - aria-buffer.test.ts (25 tests): LRU + Eviction + Buffer Pool
  - aria-wal-mvcc.test.ts (22 tests): WAL 编解码 + MVCC 事务
  - aria-compress.test.ts (11 tests): LZ4 + Merge Iterator
  - aria.test.ts (80 tests): AriaEngine 集成 + 边界测试

- Bug 修复: LRUList size 跟踪、WAL 缓冲区越界、ColumnEncoding 导入
- 全面更新 README.md + site/ 站点文件 (index/docs/demo)

src/engine/aria/store/file_manager.ts

@@ -0,0 +1,112 @@
+/**
+ * AriaEngine File Manager — 页面文件管理 + PageIO 实现
+ * @module engine/aria/store/file_manager
+ *
+ * 负责管理页面文件的生命周期：分配/释放页面 ID，读写页面。
+ */
+
+import type { IStorageBackend } from './backend';
+import type { PageIO } from '../buffer/pool';
+import { PAGE_SIZE, PageType } from '../types';
+import { initPageHeader } from '../page/header';
+
+// ---------------------------------------------------------------------------
+//  FileManager (implements PageIO)
+// ---------------------------------------------------------------------------
+
+export class FileManager implements PageIO {
+  private backend: IStorageBackend;
+  private nextPageId = 0;
+  private metaLoaded = false;
+  private dbName = '';
+
+  constructor(backend: IStorageBackend) {
+    this.backend = backend;
+  }
+
+  /** 初始化：从存储中读取元数据 */
+  async init(dbName: string): Promise<void> {
+    this.dbName = dbName;
+    // 读取 nextPageId
+    const meta = await this.backend.read('__aria_meta');
+    if (meta) {
+      const view = new DataView(meta);
+      this.nextPageId = view.getUint32(0, false);
+    } else {
+      this.nextPageId = 1;
+      await this.saveMeta();
+    }
+    this.metaLoaded = true;
+  }
+
+  // ---- PageIO ----
+
+  async readPage(pageId: number): Promise<ArrayBuffer | null> {
+    const key = `pg_${pageId}`;
+    const data = await this.backend.read(key);
+    if (!data) {
+      // 第一次访问：创建新页面
+      return this.createEmptyPage(pageId, PageType.DATA);
+    }
+
+    // 确保大小正确
+    if (data.byteLength < PAGE_SIZE) {
+      const padded = new ArrayBuffer(PAGE_SIZE);
+      new Uint8Array(padded).set(new Uint8Array(data));
+      return padded;
+    }
+
+    return data;
+  }
+
+  async writePage(pageId: number, data: ArrayBuffer): Promise<void> {
+    const key = `pg_${pageId}`;
+    await this.backend.write(key, data);
+  }
+
+  async allocatePageId(): Promise<number> {
+    const id = this.nextPageId++;
+    await this.saveMeta();
+    return id;
+  }
+
+  async freePageId(_pageId: number): Promise<void> {
+    // 简化实现：不回收 pageId
+    const key = `pg_${_pageId}`;
+    await this.backend.delete(key);
+  }
+
+  // ---- 表页面分配 ----
+
+  /**
+   * 分配一个新的表元数据页面。
+   */
+  async allocateTableRootPage(): Promise<number> {
+    const pageId = await this.allocatePageId();
+    const data = new ArrayBuffer(PAGE_SIZE);
+    initPageHeader(data, pageId, PageType.META);
+    await this.writePage(pageId, data);
+    return pageId;
+  }
+
+  // ---- 辅助 ----
+
+  private async saveMeta(): Promise<void> {
+    const buf = new ArrayBuffer(8);
+    new DataView(buf).setUint32(0, this.nextPageId, false);
+    await this.backend.write('__aria_meta', buf);
+  }
+
+  private createEmptyPage(pageId: number, type: PageType): ArrayBuffer {
+    const buf = new ArrayBuffer(PAGE_SIZE);
+    initPageHeader(buf, pageId, type);
+    return buf;
+  }
+
+  /** 清空所有数据 */
+  async clearAll(): Promise<void> {
+    await this.backend.clear();
+    this.nextPageId = 1;
+    await this.saveMeta();
+  }
+}