feat(table): route merge_insert through the MemWAL LSM write path (#3354)

## Summary

When an `LsmWriteSpec` is installed on a table (#3396), `merge_insert`
upsert
calls are dispatched through Lance's MemWAL `ShardWriter` (LSM-style
append)
instead of the standard merge path.

- **`use_lsm_write`** — a `merge_insert` builder option, default `true`;
set it
  `false` to use the standard path for a call even when a spec is set.
- **`assume_pre_sharded`** — a `merge_insert` builder option, default
`false`;
  skips the per-row shard check and routes by the first row only.
- **`close_lsm_writers`** — drains and closes the table's cached MemWAL
shard
  writers.
- The `merge_insert` **`on`** columns default to, and are validated
against,
  the table's unenforced primary key.
- Shard writers are cached alongside the dataset (in
  `DatasetConsistencyWrapper`) and reused for the session.
- `MergeResult` gains **`num_rows`** — on the LSM path the insert/update
  breakdown is unknown until compaction, so only the total is reported.

Routing covers all three sharding strategies — bucket (murmur3,
Iceberg-compatible), identity, and unsharded. Each `merge_insert` call
targets
a single shard; the whole input is collected and validated before a
single
atomic `ShardWriter::put`, so a validation failure leaves the MemWAL
untouched.

Bindings: Python (`merge_insert(...).use_lsm_write(...)` /
`.assume_pre_sharded(...)`, `Table.close_lsm_writers`) and TypeScript
(`mergeInsert(...).useLsmWrite(...)` / `.assumePreSharded(...)`,
`Table.closeLsmWriters`).

## Context

Reconstructed from the original #3354 branch onto current `main`: the
branch
predated the #3394 (unenforced primary key) / #3396 (`LsmWriteSpec`)
split and
has been rebuilt on that merged foundation. Depends on Lance
`v7.0.0-beta.13`.

The MemWAL read path (reading un-flushed shard data back into queries)
and
remote (LanceDB Cloud) LSM support are follow-ups.

---------

Co-authored-by: Jack Ye <yezhaoqin@gmail.com>

nodejs/src/table.rs

@@ -391,6 +391,11 @@ impl Table {
             .default_error()
     }
 
+    #[napi(catch_unwind)]
+    pub async fn close_lsm_writers(&self) -> napi::Result<()> {
+        self.inner_ref()?.close_lsm_writers().await.default_error()
+    }
+
     #[napi(catch_unwind)]
     pub async fn version(&self) -> napi::Result<i64> {
         self.inner_ref()?
@@ -940,6 +945,7 @@ pub struct MergeResult {
     pub num_updated_rows: i64,
     pub num_deleted_rows: i64,
     pub num_attempts: i64,
+    pub num_rows: i64,
 }
 
 impl From<lancedb::table::MergeResult> for MergeResult {
@@ -950,6 +956,7 @@ impl From<lancedb::table::MergeResult> for MergeResult {
             num_updated_rows: value.num_updated_rows as i64,
             num_deleted_rows: value.num_deleted_rows as i64,
             num_attempts: value.num_attempts as i64,
+            num_rows: value.num_rows as i64,
         }
     }
 }