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>

python/python/tests/test_index.py

@@ -226,7 +226,6 @@ async def test_create_vector_index(some_table: AsyncTable):
     assert stats.num_indexed_rows == await some_table.count_rows()
     assert stats.num_unindexed_rows == 0
     assert stats.num_indices == 1
-    assert stats.loss >= 0.0
 
 
 @pytest.mark.asyncio
@@ -250,7 +249,6 @@ async def test_create_4bit_ivfpq_index(some_table: AsyncTable):
     assert stats.num_indexed_rows == await some_table.count_rows()
     assert stats.num_unindexed_rows == 0
     assert stats.num_indices == 1
-    assert stats.loss >= 0.0
 
 
 @pytest.mark.asyncio