refactor!: rename LSM APIs to MemWAL and align SSTable/table shard naming (#4075)

## What

Renames the MemWAL surface in two passes, across Rust, Python,
TypeScript, and Java, including the wire format.

### 1. MemWAL APIs drop "LSM"

The write path these APIs drive is Lance's MemWAL; "LSM" described its
shape rather than naming it. Turning it on and off now reads as enabling
a feature rather than installing a spec.

| Before | After |
|---|---|
| `set_lsm_write_spec` | `enable_memwal` |
| `unset_lsm_write_spec` | `disable_memwal` |
| `get_lsm_write_spec` | `get_memwal_spec` |
| `flush_lsm` / `compact_lsm` / `checkpoint_lsm` | `flush_memwal` /
`compact_memwal` / `checkpoint_memwal` |
| `get_lsm_stats` | `get_memwal_stats` |
| `close_lsm_writers` | `close_memwal_writers` |
| `use_lsm` (query, merge insert) | `use_memwal` |
| `LsmWriteSpec` / `LsmStats` | `MemWalWriteSpec` / `MemWalStats` |
| Java `LanceDbTableLsm` | `LanceDbTableMemWal` |

TypeScript and Java use camelCase (`enableMemWal`, `getMemWalStats`,
...); the napi methods set `js_name` so the generated names keep the
`MemWal` casing. flush/compact/checkpoint keep a `_memwal` suffix so
`compact_memwal` is not confused with `compact_files` / `optimize`,
which compact the base table.

Internal names (`LsmMode`, `LsmScanner`, the `merge::lsm` / `query::lsm`
modules) are unchanged.

### 2. Stats use SSTable and table shard

Aligns the stats surface with the MemWAL naming settled upstream in
lance-format/lance#7943 and #7957, where the persisted unit became an
**SSTable**.

**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 already wrong for
`identity` and `year` sharding, which produce shards and no buckets at
all.

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

Docs and comments follow suit: "seal" → "freeze", "L0" / "flushed
generations" → SSTables, "bucket" (meaning a shard) → table shard.

`MemWalWriteSpec::bucket`, `num_buckets`, and the `"Sealed"` shard
status value are **unchanged** — those name the sharding transform or a
server value, not the shard.

## Compatibility

Breaking, for an experimental API. The wire format changes with the
SDKs, so this must roll out together with the matching server change:

- Routes: `/set_lsm_write_spec/` → `/enable_memwal/`,
`/unset_lsm_write_spec/` → `/disable_memwal/`, `/get_lsm_write_spec/` →
`/get_memwal_spec/`, `/flush_lsm/` → `/flush_memwal/`, `/compact_lsm/` →
`/compact_memwal/`, `/get_lsm_stats/` → `/get_memwal_stats/`
- Response keys: `lsm_write_spec` → `memwal_spec`, `lsm_stats` →
`memwal_stats`, `buckets` → `table_shards`, `generations` → `sstables`
- Request fields: `include_generation_rows` → `include_sstable_rows`;
`use_lsm` → `use_memwal` on query and merge-insert bodies

## Validation

- Rust: `cargo test --features remote -p lancedb --lib --tests` — 1467
lib tests and all integration test targets pass; `cargo clippy` clean
- Python: rebuilt the extension; `test_remote_db`, `test_memwal_spec`,
`test_merge_insert_lsm`, `test_query`, `test_permutation`, `test_table`
— 655 passed; `ruff format` / `ruff check` clean
- TypeScript: `pnpm build`; `table`, `merge`, `query` tests — 593
passed; `biome check` clean; `docs/src/js` regenerated with `npm run
docs`
- Java: `mvn test` — 37 passed (Spotless applied with JDK 11; the pinned
google-java-format does not run on JDK 21)

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

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/src/js/interfaces/MemWalWriteSpec.md

@@ -0,0 +1,68 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / MemWalWriteSpec
+
+# Interface: MemWalWriteSpec
+
+Specification selecting Lance's MemWAL LSM-style write path for
+`mergeInsert`.
+
+`specType` is `"bucket"`, `"identity"`, or `"unsharded"`. For `"bucket"`,
+`column` and `numBuckets` are required; for `"identity"`, `column` is
+required and must be a deterministic function of the unenforced primary
+key (every row with a given primary key must always produce the same
+`column` value, or upserts of that key can land in different shards and a
+stale version can win).
+
+## Properties
+
+### column?
+
+```ts
+optional column: string;
+```
+
+Bucket and identity variants: the sharding column.
+
+***
+
+### maintainedIndexes?
+
+```ts
+optional maintainedIndexes: string[];
+```
+
+Indexes the MemWAL keeps up to date. Omit for every index the table has,
+including ones created later. Pass `[]` for none.
+
+***
+
+### numBuckets?
+
+```ts
+optional numBuckets: number;
+```
+
+Bucket variant: the number of buckets, in `[1, 1024]`.
+
+***
+
+### specType
+
+```ts
+specType: "bucket" | "identity" | "unsharded";
+```
+
+One of `"bucket"`, `"identity"`, or `"unsharded"`.
+
+***
+
+### writerConfigDefaults?
+
+```ts
+optional writerConfigDefaults: Record<string, string>;
+```
+
+Default `ShardWriter` configuration recorded in the MemWAL index.