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() });
```

docs/src/js/classes/Index.md

@@ -57,6 +57,24 @@ block size may be added in the future.
 
 ***
 
+### fm()
+
+```ts
+static fm(): Index
+```
+
+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.
+
+#### Returns
+
+[`Index`](Index.md)
+
+***
+
 ### fts()
 
 ```ts