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`

docs/src/js/classes/Table.md

@@ -137,6 +137,20 @@ containing the new version number of the table after altering the columns.
 
 ***
 
+### blobColumns()
+
+```ts
+abstract blobColumns(): Promise<string[]>
+```
+
+Blob v2 columns, including nested dotted paths.
+
+#### Returns
+
+`Promise`&lt;`string`[]&gt;
+
+***
+
 ### branches()
 
 ```ts
@@ -499,6 +513,54 @@ Drop an index from the table.
 
 ***
 
+### fetchBlobFiles()
+
+```ts
+abstract fetchBlobFiles(column, rowIds): Promise<(null | BlobFile)[]>
+```
+
+Opens lazy blob handles for `column` at the given row IDs using the
+table's current checkout.
+
+Preserves input order, duplicates, and nulls. Use this for large payloads.
+See [Table.fetchBlobs](Table.md#fetchblobs) for row-ID validity across versions.
+
+#### Parameters
+
+* **column**: `string`
+
+* **rowIds**: readonly (`number` \| `bigint`)[]
+
+#### Returns
+
+`Promise`&lt;(`null` \| [`BlobFile`](BlobFile.md))[]&gt;
+
+***
+
+### fetchBlobs()
+
+```ts
+abstract fetchBlobs(column, rowIds): Promise<(null | Buffer)[]>
+```
+
+Bytes for `column` at row IDs from [Query.withRowId](Query.md#withrowid).
+
+Reads the table's current checkout. IDs from another version can fail after
+compaction unless stable row ids are enabled. Results keep input order and
+duplicates. Null blobs are `null`. Empty blobs are empty buffers.
+
+#### Parameters
+
+* **column**: `string`
+
+* **rowIds**: readonly (`number` \| `bigint`)[]
+
+#### Returns
+
+`Promise`&lt;(`null` \| `Buffer`)[]&gt;
+
+***
+
 ### flushLsm()
 
 ```ts

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`

docs/src/js/globals.md

@@ -19,6 +19,7 @@
 ## Classes
 
 - [AutoQuery](classes/AutoQuery.md)
+- [BlobFile](classes/BlobFile.md)
 - [BooleanQuery](classes/BooleanQuery.md)
 - [BoostQuery](classes/BoostQuery.md)
 - [BranchContents](classes/BranchContents.md)
@@ -143,6 +144,7 @@
 
 - [AnalyzePlanDistributedMetrics](type-aliases/AnalyzePlanDistributedMetrics.md)
 - [BaseTokenizer](type-aliases/BaseTokenizer.md)
+- [BlobOptions](type-aliases/BlobOptions.md)
 - [Data](type-aliases/Data.md)
 - [DataLike](type-aliases/DataLike.md)
 - [FieldLike](type-aliases/FieldLike.md)
@@ -158,9 +160,11 @@
 ## Functions
 
 - [RecordBatchIterator](functions/RecordBatchIterator.md)
+- [blob](functions/blob.md)
 - [connect](functions/connect.md)
 - [connectNamespace](functions/connectNamespace.md)
 - [instrumentLanceDbMetrics](functions/instrumentLanceDbMetrics.md)
+- [isBlobField](functions/isBlobField.md)
 - [makeArrowTable](functions/makeArrowTable.md)
 - [packBits](functions/packBits.md)
 - [permutationBuilder](functions/permutationBuilder.md)

docs/src/js/type-aliases/BlobOptions.md

@@ -0,0 +1,48 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BlobOptions
+
+# Type Alias: BlobOptions
+
+```ts
+type BlobOptions: object;
+```
+
+## Type declaration
+
+### dedicatedSizeThreshold?
+
+```ts
+optional dedicatedSizeThreshold: number;
+```
+
+Max payload bytes stored in a packed sidecar before a dedicated file. Must
+be a positive safe integer.
+
+### inlineSizeThreshold?
+
+```ts
+optional inlineSizeThreshold: number;
+```
+
+Max payload bytes kept inline in the data file. Zero is allowed. Must be a
+safe integer.
+
+### nullable?
+
+```ts
+optional nullable: boolean;
+```
+
+Defaults to true.
+
+### packFileSizeThreshold?
+
+```ts
+optional packFileSizeThreshold: number;
+```
+
+Max bytes in one packed sidecar before starting another. Must be a positive
+safe integer.