feat: support FM-Index scalar index for substring search (#3532)

Adds an FM-Index — a scalar index over string and binary columns that
accelerates substring search (`contains(col, 'needle')`), distinct from
the tokenized `FTS` index — across the Rust core and the Python and
TypeScript bindings.

## Rust

- `Index::Fm(FmIndexBuilder)` and `IndexType::Fm`.
- `make_index_params` maps `Index::Fm` to Lance's
`ScalarIndexParams::for_builtin(BuiltinIndexType::Fm)`.
- `supported_fm_data_type` validates
`Utf8`/`LargeUtf8`/`Binary`/`LargeBinary` columns.
- `list_indices` round-trips the type (`"Fm"` → `IndexType::Fm`); the
remote wire type is `"FM"`.

## Python

Adds `lancedb.index.Fm`, accepted by `create_index`:

```python
from lancedb.index import Fm

await tbl.create_index("text", config=Fm())
```

## TypeScript

Adds the `Index.fm()` factory:

```ts
await tbl.createIndex("text", { config: Index.fm() });
```

nodejs/__test__/table.test.ts

@@ -1431,6 +1431,20 @@ describe("When creating an index", () => {
     expect(fs.readdirSync(indexDir)).toHaveLength(1);
   });
 
+  test("create an FM index", async () => {
+    // FM-Index accelerates substring search on a string/binary column.
+    const db = await connect(tmpDir.name);
+    const fmTbl = await db.createTable("fm_table", [
+      { id: 0, text: "hello world" },
+      { id: 1, text: "foo bar" },
+    ]);
+    await fmTbl.createIndex("text", {
+      config: Index.fm(),
+    });
+    const indexDir = path.join(tmpDir.name, "fm_table.lance", "_indices");
+    expect(fs.readdirSync(indexDir)).toHaveLength(1);
+  });
+
   test("should be able to get index stats", async () => {
     await tbl.createIndex("id");
 

nodejs/lancedb/indices.ts

@@ -702,6 +702,17 @@ export class Index {
     return new Index(LanceDbIndex.labelList());
   }
 
+  /**
+   * Create an FM-Index.
+   *
+   * An FM-Index is a scalar index on string or binary columns that accelerates
+   * substring search, i.e. `contains(col, 'needle')`. Unlike the tokenized
+   * full-text-search index, it matches arbitrary substrings of the raw bytes.
+   */
+  static fm() {
+    return new Index(LanceDbIndex.fm());
+  }
+
   /**
    * Create a full text search index
    *

nodejs/src/index.rs

@@ -4,7 +4,7 @@
 use std::sync::Mutex;
 
 use lancedb::index::Index as LanceDbIndex;
-use lancedb::index::scalar::{BTreeIndexBuilder, FtsIndexBuilder};
+use lancedb::index::scalar::{BTreeIndexBuilder, FmIndexBuilder, FtsIndexBuilder};
 use lancedb::index::vector::{
     IvfFlatIndexBuilder, IvfHnswPqIndexBuilder, IvfHnswSqIndexBuilder, IvfPqIndexBuilder,
     IvfRqIndexBuilder,
@@ -143,6 +143,13 @@ impl Index {
         }
     }
 
+    #[napi(factory)]
+    pub fn fm() -> Self {
+        Self {
+            inner: Mutex::new(Some(LanceDbIndex::Fm(FmIndexBuilder::default()))),
+        }
+    }
+
     #[napi(factory)]
     #[allow(clippy::too_many_arguments)]
     pub fn fts(