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/src/index.rs

@@ -7,7 +7,7 @@ use lancedb::index::vector::{
 };
 use lancedb::index::{
     Index as LanceDbIndex,
-    scalar::{BTreeIndexBuilder, FtsIndexBuilder},
+    scalar::{BTreeIndexBuilder, FmIndexBuilder, FtsIndexBuilder},
 };
 use pyo3::IntoPyObject;
 use pyo3::types::PyStringMethods;
@@ -38,6 +38,7 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
             "BTree" => Ok(LanceDbIndex::BTree(BTreeIndexBuilder::default())),
             "Bitmap" => Ok(LanceDbIndex::Bitmap(Default::default())),
             "LabelList" => Ok(LanceDbIndex::LabelList(Default::default())),
+            "Fm" => Ok(LanceDbIndex::Fm(FmIndexBuilder::default())),
             "FTS" => {
                 let params = source.extract::<FtsParams>()?;
                 let inner_opts = FtsIndexBuilder::default()
@@ -183,7 +184,7 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
                 Ok(LanceDbIndex::IvfHnswFlat(hnsw_flat_builder))
             }
             not_supported => Err(PyValueError::new_err(format!(
-                "Invalid index type '{}'.  Must be one of BTree, Bitmap, LabelList, FTS, IvfPq, IvfSq, IvfHnswPq, IvfHnswSq, or IvfHnswFlat",
+                "Invalid index type '{}'.  Must be one of BTree, Bitmap, LabelList, Fm, FTS, IvfPq, IvfSq, IvfHnswPq, IvfHnswSq, or IvfHnswFlat",
                 not_supported
             ))),
         }