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>

Cargo.lock

@@ -8468,6 +8468,12 @@ dependencies = [
  "digest 0.11.3",
 ]
 
+[[package]]
+name = "sha1_smol"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbfa15b3dddfee50a0fff136974b3e1bde555604ba463834a7eb7deb6417705d"
+
 [[package]]
 name = "sha2"
 version = "0.10.9"
@@ -9691,6 +9697,7 @@ dependencies = [
  "getrandom 0.4.2",
  "js-sys",
  "serde_core",
+ "sha1_smol",
  "wasm-bindgen",
 ]