refactor!: drop unused loss field from IndexStatistics (#3496)

BREAKING CHANGE: direct Rust users lose the `IndexStatistics::loss`
field. Python and Node.js consumers are unaffected in practice for
remote tables (the value was always `None`/absent), but the attribute is
gone for local tables too.

`IndexStatistics::loss` was local-only — LanceDB Cloud never returned
it, so
`RemoteTable::index_stats` always set `loss: None`. It's vestigial; this
removes it.

- Remove `loss` from `IndexStatistics` and the internal `IndexMetadata`
in `rust/lancedb/src/index.rs`, plus the summing logic in
`NativeTable::index_stats`.
- Drop `loss` from the Python and Node.js bindings (and their
tests/docs).

Fixes #3493

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nodejs/__test__/table.test.ts

@@ -721,7 +721,7 @@ describe("When creating an index", () => {
       columns: ["vec"],
     });
     const stats = await tbl.indexStats("vec_idx");
-    expect(stats?.loss).toBeDefined();
+    expect(stats).toBeDefined();
 
     // Search without specifying the column
     let rst = await tbl
@@ -1150,7 +1150,6 @@ describe("When creating an index", () => {
     expect(stats?.distanceType).toBeUndefined();
     expect(stats?.indexType).toEqual("BTREE");
     expect(stats?.numIndices).toEqual(1);
-    expect(stats?.loss).toBeUndefined();
   });
 
   test("when getting stats on non-existent index", async () => {