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

docs/src/js/globals.md

@@ -60,7 +60,6 @@
 - [BranchDiff](interfaces/BranchDiff.md)
 - [BranchIndexSummary](interfaces/BranchIndexSummary.md)
 - [BranchRowCountSummary](interfaces/BranchRowCountSummary.md)
-- [BucketStats](interfaces/BucketStats.md)
 - [CherryPickError](interfaces/CherryPickError.md)
 - [CherryPickPreview](interfaces/CherryPickPreview.md)
 - [CherryPickResult](interfaces/CherryPickResult.md)
@@ -87,7 +86,6 @@
 - [FtsToken](interfaces/FtsToken.md)
 - [FullTextQuery](interfaces/FullTextQuery.md)
 - [FullTextSearchOptions](interfaces/FullTextSearchOptions.md)
-- [GenerationStats](interfaces/GenerationStats.md)
 - [HnswPqOptions](interfaces/HnswPqOptions.md)
 - [HnswSqOptions](interfaces/HnswSqOptions.md)
 - [IndexConfig](interfaces/IndexConfig.md)
@@ -126,7 +124,9 @@
 - [SplitHashOptions](interfaces/SplitHashOptions.md)
 - [SplitRandomOptions](interfaces/SplitRandomOptions.md)
 - [SplitSequentialOptions](interfaces/SplitSequentialOptions.md)
+- [SsTableStats](interfaces/SsTableStats.md)
 - [TableNamesOptions](interfaces/TableNamesOptions.md)
+- [TableShardStats](interfaces/TableShardStats.md)
 - [TableStatistics](interfaces/TableStatistics.md)
 - [TimeoutConfig](interfaces/TimeoutConfig.md)
 - [TlsConfig](interfaces/TlsConfig.md)

docs/src/js/interfaces/GenerationStats.md

@@ -1,40 +0,0 @@
-[**@lancedb/lancedb**](../README.md) • **Docs**
-
-***
-
-[@lancedb/lancedb](../globals.md) / GenerationStats
-
-# Interface: GenerationStats
-
-One flushed L0 generation.
-
-## Properties
-
-### bytes
-
-```ts
-bytes: number;
-```
-
-On-disk size of the generation.
-
-***
-
-### generation
-
-```ts
-generation: number;
-```
-
-The generation number. Increases as memtables are sealed into L0.
-
-***
-
-### rows?
-
-```ts
-optional rows: number;
-```
-
-Present only when `includeGenerationRows` was requested. Off by default
-because each count opens an uncached Lance dataset.

docs/src/js/interfaces/LsmStats.md

@@ -6,17 +6,17 @@
 
 # Interface: LsmStats
 
-Live per-bucket LSM state, as returned by `Table#getLsmStats`.
+Live per-table-shard LSM state, as returned by `Table#getLsmStats`.
 
-Nothing here is derived: sums and differences (total L0 bytes, WAL lag) are
+Nothing here is derived: sums and differences (total SSTable bytes, WAL lag) are
 the caller's to compute.
 
 ## Properties
 
-### buckets
+### tableShards
 
 ```ts
-buckets: BucketStats[];
+tableShards: TableShardStats[];
 ```
 
-One entry per bucket backing this table.
+One entry per table shard backing this table.

docs/src/js/interfaces/SsTableStats.md

@@ -0,0 +1,40 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / SsTableStats
+
+# Interface: SsTableStats
+
+One SSTable.
+
+## Properties
+
+### bytes
+
+```ts
+bytes: number;
+```
+
+On-disk size of the SSTable.
+
+***
+
+### generation
+
+```ts
+generation: number;
+```
+
+The generation number. Increases as memtables are frozen into SSTables.
+
+***
+
+### rows?
+
+```ts
+optional rows: number;
+```
+
+Present only when `includeSstableRows` was requested. Off by default
+because each count opens an uncached Lance dataset.

docs/src/js/interfaces/TableShardStats.md

@@ -2,12 +2,12 @@
 
 ***
 
-[@lancedb/lancedb](../globals.md) / BucketStats
+[@lancedb/lancedb](../globals.md) / TableShardStats
 
-# Interface: BucketStats
+# Interface: TableShardStats
 
-Live state of one bucket. A table is N buckets on one node; flattening to a
-single number hides the one hot bucket that is usually why someone opened
+Live state of one table shard. A table is N table shards on one node; flattening to a
+single number hides the one hot table shard that is usually why someone opened
 this endpoint.
 
 ## Properties
@@ -18,7 +18,7 @@ this endpoint.
 compacting: boolean;
 ```
 
-Whether a pass owns this bucket's compaction latch right now. Says *a*
+Whether a pass owns this table shard's compaction latch right now. Says *a*
 driver is running, not *whose*, and the latch is held from dispatch —
 including while the pass queues for a pod-wide compactor permit. Read it
 as "do not pile on", never as "mine is progressing".
@@ -35,13 +35,13 @@ The generation the active memtable will become.
 
 ***
 
-### generations
+### sstables
 
 ```ts
-generations: GenerationStats[];
+sstables: SsTableStats[];
 ```
 
-Flushed L0 generations not yet merged into the base table.
+SSTables not yet merged into the base table.
 
 ***
 
@@ -61,7 +61,7 @@ Version of the shard manifest these numbers were read from.
 optional memtables: MemtableStats[];
 ```
 
-Oldest first, active last. Absent for a `"Sealed"` bucket, whose
+Oldest first, active last. Absent for a `"Sealed"` table shard, whose
 in-memory state is torn down.
 
 ***
@@ -82,7 +82,7 @@ WAL position replay resumes from.
 shardId: string;
 ```
 
-The shard this bucket writes.
+The shard this table shard writes.
 
 ***