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>

python/src/lib.rs

@@ -16,8 +16,8 @@ use query::{FTSQuery, HybridQuery, Query, VectorQuery};
 use session::Session;
 use table::{
     AddColumnsResult, AddResult, AlterColumnsResult, DeleteResult, DropColumnsResult, FtsToken,
-    FunctionErrorFragment, FunctionErrorRecord, FunctionErrors, MemWalWriteSpec, MergeResult,
-    PyBlobFile, RefreshColumnResult, RefreshMaterializedViewResult, Table,
+    FunctionErrorFragment, FunctionErrorRecord, FunctionErrors, MergeResult, PyBlobFile,
+    RTEWriteSpec, RefreshColumnResult, RefreshMaterializedViewResult, Table,
     UpdateFieldMetadataResult, UpdateResult,
 };
 
@@ -104,7 +104,7 @@ pub fn _lancedb(py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<UpdateFieldMetadataResult>()?;
     m.add_class::<AddResult>()?;
     m.add_class::<MergeResult>()?;
-    m.add_class::<MemWalWriteSpec>()?;
+    m.add_class::<RTEWriteSpec>()?;
     m.add_class::<DeleteResult>()?;
     m.add_class::<DropColumnsResult>()?;
     m.add_class::<UpdateResult>()?;