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/functions/blob.md

@@ -0,0 +1,55 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / blob
+
+# Function: blob()
+
+```ts
+function blob(name, options): Field
+```
+
+Declares a `lance.blob.v2` column.
+
+Query results are descriptors, not payload bytes. Use [Table.fetchBlobs](../classes/Table.md#fetchblobs)
+or [Table.fetchBlobFiles](../classes/Table.md#fetchblobfiles) to read bytes.
+
+## Parameters
+
+* **name**: `string`
+
+* **options**: [`BlobOptions`](../type-aliases/BlobOptions.md) = `{}`
+
+## Returns
+
+`Field`
+
+## Example
+
+```ts
+import { readFile } from "node:fs/promises";
+import { Field, Int64, Schema } from "apache-arrow";
+import { blob, connect } from "@lancedb/lancedb";
+
+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 size = handle!.size();
+const header = await handle!.readRange(0n, size < 65536n ? size : 65536n);
+```

docs/src/js/functions/isBlobField.md

@@ -0,0 +1,22 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / isBlobField
+
+# Function: isBlobField()
+
+```ts
+function isBlobField(field): boolean
+```
+
+Checks for the `lance.blob.v2` extension marker. Does not validate the
+field's storage type.
+
+## Parameters
+
+* **field**: `Field`<`any`>
+
+## Returns
+
+`boolean`