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>

docs/src/js/interfaces/IndexStatistics.md

@@ -30,17 +30,6 @@ The type of the index
 
 ***
 
-### loss?
-
-```ts
-optional loss: number;
-```
-
-The KMeans loss value of the index,
-it is only present for vector indices.
-
-***
-
 ### numIndexedRows
 
 ```ts