refactor(lsm): rename LSM stats to SSTable and table shard

Aligns the LSM stats surface with the MemWAL naming settled upstream in
lance-format/lance#7943 and #7957, where the persisted unit became an
SSTable. Two terms in this API predate that pass.

**Generation -> SSTable.** A `GenerationStats` describes one flushed
MemTable, which is an SSTable. The generation *number* is kept — an
SSTable is identified by its generation — so only the noun moved.

**Bucket -> table shard.** Each entry is one MemWAL shard. "Bucket" names
only the hash sharding transform, so it was wrong for `identity` and
`year` sharding, which produce shards and no buckets at all.

| Before | After |
|---|---|
| `GenerationStats` | `SsTableStats` |
| `BucketStats` | `TableShardStats` |
| `LsmStats.buckets` | `LsmStats.table_shards` |
| `BucketStats.generations` | `TableShardStats.sstables` |
| `include_generation_rows` | `include_sstable_rows` |
| `newest_generation` | `newest_sstable_generation` |
| `outstanding_generations` | `outstanding_sstables` |

Applied across Rust, Python, TypeScript, and Java, including the
`get_lsm_stats` JSON field names. `LsmWriteSpec::Bucket` and `num_buckets`
are unchanged — those name the sharding transform, not the shard.

## Compatibility

Breaking for the MemWAL LSM stats API, which is experimental and paired
with a server that renames the same fields. The JSON keys `table_shards`
and `sstables` must roll out together with the WAL server change.

## Validation

- `cargo check -p lancedb --all-features`, `cargo fmt --all`
- `ruff format --check` and `ruff check` on the touched Python
- `biome check` on the touched TypeScript

The typedoc markdown under `docs/src/js` was updated by hand, not
regenerated: `npm run docs` needs the napi-built `./native` types. Worth
running `npm run docs` on this branch to confirm the generator agrees.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgVf5C2yRbwK6pe1aMYkmg

docs/src/js/classes/Table.md

@@ -221,7 +221,7 @@ abstract checkpointLsm(): Promise<void>
 
 Converge this table's LSM write path into its base table.
 
-Seals once, then triggers compaction and polls until the L0 that existed
+Freezes once, then triggers compaction and polls until the SSTables that existed
 at the start is gone. The target set is fixed at the start, so
 generations created *during* the checkpoint are ignored — that is what
 lets it terminate under write load, and what makes it best-effort: it
@@ -289,7 +289,7 @@ It is a no-op when no writers are cached.
 abstract compactLsm(): Promise<void>
 ```
 
-Trigger a background L0 → base compaction pass per bucket.
+Trigger a background SSTable compaction pass per table shard.
 
 Returns once the passes are *dispatched*, not once they finish — watch
 [Table#getLsmStats](Table.md#getlsmstats) for progress, or use
@@ -505,7 +505,7 @@ Drop an index from the table.
 abstract flushLsm(): Promise<void>
 ```
 
-Seal every bucket's active memtable into a new L0 generation.
+Freeze every table shard's active memtable into a new SSTable.
 
 Returns once the seal is committed. Sealing an empty memtable is a no-op,
 so this is safe to call repeatedly.
@@ -519,10 +519,10 @@ so this is safe to call repeatedly.
 ### getLsmStats()
 
 ```ts
-abstract getLsmStats(includeGenerationRows?): Promise<undefined | LsmStats>
+abstract getLsmStats(includeSstableRows?): Promise<undefined | LsmStats>
 ```
 
-Read live per-bucket LSM state.
+Read live per-table-shard LSM state.
 
 Answers "how far behind is my fresh tier", "which bucket is hot", and
 "why is my fresh-tier vector search brute-force". Mutates no table state.
@@ -531,8 +531,8 @@ Resolves to `undefined` only when the LSM write path is not enabled.
 
 #### Parameters
 
-* **includeGenerationRows?**: `boolean`
-    Also count rows per L0 generation.
+* **includeSstableRows?**: `boolean`
+    Also count rows per SSTable.
     Off by default because each count opens an uncached Lance dataset.
 
 #### Returns