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() });
```

python/python/tests/test_index.py

@@ -20,6 +20,7 @@ from lancedb.index import (
     IvfRq,
     Bitmap,
     LabelList,
+    Fm,
     HnswPq,
     HnswSq,
     HnswFlat,
@@ -203,6 +204,16 @@ async def test_create_fixed_size_binary_index(some_table: AsyncTable):
     assert indices[0].columns == ["fsb"]
 
 
+@pytest.mark.asyncio
+async def test_create_fm_index(some_table: AsyncTable):
+    # FM-Index accelerates substring search on string/binary columns.
+    await some_table.create_index("data", config=Fm())
+    indices = await some_table.list_indices()
+    assert len(indices) == 1
+    assert indices[0].index_type == "Fm"
+    assert indices[0].columns == ["data"]
+
+
 @pytest.mark.asyncio
 async def test_create_bitmap_index(some_table: AsyncTable):
     await some_table.create_index("id", config=Bitmap())