feat(node): add blob v2 fetch and field helpers (#4155)

this PR blob v2 field helpers and reads to the Node SDK.

`blob()` marks a field as blob v2 and lets you set the storage
thresholds. Inputs can be bytes, a URI, or a data/uri struct.

Queries return descriptors. `fetchBlobs()` reads the bytes by row ID,
and `fetchBlobFiles()` gives you lazy handles for full or range reads.
`blobColumns()` lists the blob fields, including nested ones.

Fetch uses the table’s current checkout. It preserves order, duplicates,
and nulls. Holding row IDs across compaction still requires stable row
IDs.

```javascript
const db = await connect("./data");
const video = await readFile("clip.mp4");
const table = await db.createTable(
  "videos",
  [{ id: 1n, video }],
  {
    schema: new Schema([
      new Field("id", new Int64()),
      blob("video"),
    ]),
  },
);
const rows = await table.query().select(["id"]).withRowId().toArray();
const rowIds = rows.map((row) => row._rowid as bigint);
const bytes = await table.fetchBlobs("video", rowIds);
const [handle] = await table.fetchBlobFiles("video", rowIds);
const header = await handle!.readRange(0n, 65536n);

```

### Testing

- cover input validation, thresholds, nested fields, fetch ordering,
nulls, and range reads.

docs/src/js/classes/BlobFile.md

@@ -0,0 +1,62 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BlobFile
+
+# Class: BlobFile
+
+A lazy handle to blob bytes. Create one with [Table.fetchBlobFiles](Table.md#fetchblobfiles).
+
+## Methods
+
+### read()
+
+```ts
+read(): Promise<Buffer>
+```
+
+Reads from the cursor to the end and advances the cursor.
+
+A second call returns an empty buffer. [BlobFile.readRange](BlobFile.md#readrange) does
+not move the cursor.
+
+#### Returns
+
+`Promise`&lt;`Buffer`&gt;
+
+***
+
+### readRange()
+
+```ts
+readRange(start, end): Promise<Buffer>
+```
+
+Reads the half-open byte range `[start, end)`.
+
+Fails when `end` is past the blob size. Does not move the cursor.
+
+#### Parameters
+
+* **start**: `bigint`
+
+* **end**: `bigint`
+
+#### Returns
+
+`Promise`&lt;`Buffer`&gt;
+
+***
+
+### size()
+
+```ts
+size(): bigint
+```
+
+Returns the blob size in bytes.
+
+#### Returns
+
+`bigint`