refactor!: rename MemWAL APIs to RTE (realtime engine) (#4598)

## What

Renames the user-facing MemWAL surface to **RTE** (realtime engine)
across Rust, Python, TypeScript, and Java, including the REST wire
format. This follows the LSM → MemWAL rename in #4075.

| Before | After |
|---|---|
| `MemWalWriteSpec` / `MemWalStats` | `RTEWriteSpec` / `RTEStats` |
| `enable_memwal` / `disable_memwal` / `get_memwal_spec` | `enable_rte`
/ `disable_rte` / `get_rte_spec` |
| `flush_memwal` / `compact_memwal` / `checkpoint_memwal` | `flush_rte`
/ `compact_rte` / `checkpoint_rte` |
| `get_memwal_stats` / `close_memwal_writers` | `get_rte_stats` /
`close_rte_writers` |
| `use_memwal` (query, merge insert) | `use_rte` |
| TS/Java `enableMemWal`, `useMemWal`, ... | `enableRTE`, `useRTE`, ...
(napi `js_name` set explicitly) |
| Java `LanceDbTableMemWal` | `LanceDbTableRTE` |
| `table/memwal_stats.rs` | `table/rte_stats.rs` |

Docs and comments now say "realtime engine (RTE)". Text about Lance's
underlying mechanism keeps "MemWAL" ("Lance's MemWAL", "MemWAL index").

**Left alone:** Lance-owned names (`MemWalIndexDetails`,
`DatasetMemWalExt`, `InitializeMemWal*`, `mem_wal` paths), internal
`Lsm*` names and modules, and the `does not support with_row_id` refusal
text that the hybrid fallback matches on.

## Compatibility

Breaking, for an experimental API. The wire format changes, so this must
roll out together with the matching sophon change (lancedb/sophon#8870):

- Routes: `/enable_memwal/` → `/enable_rte/`, `/disable_memwal/` →
`/disable_rte/`, `/get_memwal_spec/` → `/get_rte_spec/`,
`/get_memwal_stats/` → `/get_rte_stats/`, `/flush_memwal/` →
`/flush_rte/`, `/compact_memwal/` → `/compact_rte/`
- Response keys: `memwal_spec` → `rte_spec`, `memwal_stats` →
`rte_stats`
- Request field: `use_memwal` → `use_rte` on query and merge-insert
bodies

The SQL keyword becomes `ALTER TABLE ... SET RTE` server-side. The
guides under `docs/web` will be updated separately.

## Validation

- Rust: `cargo clippy --features remote --tests --examples` clean;
`cargo test --features remote -p lancedb --lib`: 1533 passed
- Java: `mvn test`: 37 passed (Spotless applied with JDK 11)
- Python: `ruff format` / `ruff check` clean; targeted tests
(`test_rte_spec`, `test_merge_insert_lsm`, `test_remote_db`,
`test_wal_hybrid`, `test_query`, `test_permutation`, `test_table`): 708
passed
- TypeScript: `pnpm build`; `table`, `query` tests: 606 passed; `biome
check` clean; `docs/src/js` regenerated with `pnpm run docs`

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

---------

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

java/README.md

@@ -29,34 +29,34 @@ LanceNamespace namespaceClient = LanceDbNamespaceClientBuilder.newBuilder()
     .build();
 ```
 
-## MemWAL write path
+## RTE write path
 
 Most table operations reach LanceDB through the `LanceNamespace` above, which is
-generated from the Lance Namespace specification. The MemWAL LSM routes are not part
+generated from the Lance Namespace specification. The RTE LSM routes are not part
 of that specification, so they are issued through a separate client:
 
 ```java
 import com.lancedb.LanceDbRestClient;
-import com.lancedb.LanceDbTableMemWal;
-import com.lancedb.MemWalWriteSpec;
+import com.lancedb.LanceDbTableRTE;
+import com.lancedb.RTEWriteSpec;
 
 LanceDbRestClient client = LanceDbNamespaceClientBuilder.newBuilder()
     .apiKey("your_lancedb_cloud_api_key")
     .database("your_database_name")
     .buildRestClient();
 
-LanceDbTableMemWal memWal = new LanceDbTableMemWal(client, "my_table");
+LanceDbTableRTE rte = new LanceDbTableRTE(client, "my_table");
 
-// Route future merge_insert upserts through the MemWAL, hash-bucketed by `id`.
-memWal.enableMemWal(MemWalWriteSpec.bucket("id", 16));
+// Route future merge_insert upserts through the RTE, hash-bucketed by `id`.
+rte.enableRTE(RTEWriteSpec.bucket("id", 16));
 
 // ... merge_insert traffic ...
 
 // Converge the fresh tier into the base table.
-memWal.checkpointMemWal();
+rte.checkpointRTE();
 
 // Inspect live per-table-shard state.
-memWal.getMemWalStats().ifPresent(stats -> stats.tableShards().forEach(tableShard ->
+rte.getRTEStats().ifPresent(stats -> stats.tableShards().forEach(tableShard ->
     System.out.println(tableShard.shardId() + ": " + tableShard.sstables().size() + " SSTables")));
 
 client.close();