feat: Integrate libseekdb to support embedded mode

[dengfuping]

Summary

• Add embedded mode so the SDK can run against a local seekdb instance via the native addon (@seekdb/js-bindings) instead of a remote MySQL server. Same public API: path → embedded, host → server.
• Dependencies: seekdb C ABI #135

Solution Description

• Embedded Mode Implementation Architecture

                    Client(args) / AdminClient(args)
                                    │
                    ┌───────────────┴───────────────┐
                    │ path?          │ host?       │
                    ▼                ▼             │
         ┌──────────────────┐  ┌──────────────────┐
         │ SeekdbEmbedded    │  │ SeekdbServer     │
         │ Client            │  │ Client           │
         └────────┬──────────┘  └────────┬─────────┘
                  │                      │
                  │     BaseSeekdbClient │
                  │     (collections)    │
                  │                      │
                  ▼                      ▼
         ┌──────────────────┐  ┌──────────────────┐
         │ InternalEmbedded │  │ InternalClient   │
         │ Client           │  │ (MySQL Connection)│
         └────────┬──────────┘  └──────────────────┘
                  │
                  ▼
         ┌──────────────────┐     ┌──────────────────┐
         │ @seekdb/js-      │ ──► │ libseekdb (C ABI)│
         │ bindings (native)│     │ seekdb_open,     │
         │ open→connect→    │     │ seekdb_connect, │
         │ execute          │     │ seekdb_query_*   │
         └──────────────────┘     └──────────────────┘

• Main changes:

• Client: SeekdbClient facade picks SeekdbEmbeddedClient or SeekdbServerClient from path vs host. Both extend BaseSeekdbClient and use IInternalClient for execution.

• Embedded path: New InternalEmbeddedClient implements IInternalClient with bindings: open(path) → connect(db, database) → execute(conn, sql, params). Converts bindings result to RowDataPacket[] so Collection layer is unchanged.

• Factory: Client(args) — if path → embedded; if host → server; if neither → default embedded at cwd/seekdb.db (throws if addon missing). Same for AdminClient.

• Types: SeekdbClientArgs.path? (embedded), SeekdbClientArgs.host? (server).

• Bindings: C++ already exposes open, connect, execute; no API change. Docs added for C ABI expectations (c-abi-modification-reference.md).

• Tests: packages/seekdb/tests/embedded/ — client, collection (DML/query/approximate/hybrid), data, edge-cases, embedding, examples. test-utils.ts for isolated DB dirs and cleanup.

Usage

import { SeekdbClient } from "seekdb";

const client = new SeekdbClient({
  path: "./seekdb.db",
  database: "test",
});

const collection = await client.createCollection({ name: "my_collection" });
await collection.add({
  ids: ["1", "2"],
  documents: ["Hello world", "seekdb is fast"],
});
const results = await collection.query({ queryTexts: "Hello", nResults: 5 });

Checklist

• Embedded and server collection/document behavior equivalent
• Lazy init and no-op close acceptable; no event-loop block
• Error handling (addon missing, init twice, C errors) clear
• Tests cover create/get/DML/query/edge cases; isolated DB dirs