feat: support setting LSM write spec for a table (#3396)

## Summary

Split out from #3354

Adds `LsmWriteSpec` and `Table::set_lsm_write_spec` /
`unset_lsm_write_spec` to
install and clear the spec that selects Lance's MemWAL LSM-style write
path for
`merge_insert`.

`LsmWriteSpec` offers three sharding strategies, all built on Lance's
`InitializeMemWalBuilder`:

- `LsmWriteSpec::bucket(column, num_buckets)` — hash-bucket sharding by
the
  single-column unenforced primary key.
- `LsmWriteSpec::identity(column)` — identity sharding by the raw value
of a
  scalar column.
- `LsmWriteSpec::unsharded()` — a single MemWAL shard.

Each can be refined with `with_maintained_indexes(...)` (indexes the
MemWAL
keeps up to date as rows are appended) and
`with_writer_config_defaults(...)`
(default `ShardWriter` configuration recorded in the MemWAL index, so
every
writer starts from the same defaults). All variants require the table to
have
an unenforced primary key.

- `set_lsm_write_spec` installs the spec by initializing the MemWAL
index;
`unset_lsm_write_spec` removes it (dropping the MemWAL index), reverting
to
  the standard `merge_insert` path. `unset` is idempotent.
- Bindings: Python (`LsmWriteSpec.bucket` / `.identity` / `.unsharded`,
  `set_lsm_write_spec` / `unset_lsm_write_spec`) and TypeScript
  (`setLsmWriteSpec` with `specType` `"bucket"` / `"identity"` /
  `"unsharded"`). `RemoteTable` returns `NotSupported`.

The actual `merge_insert` LSM dispatch and `ShardWriter` write path are
a
follow-up — this PR only installs and clears the spec.

python/src/lib.rs

@@ -15,8 +15,8 @@ use pyo3::{
 use query::{FTSQuery, HybridQuery, Query, VectorQuery};
 use session::Session;
 use table::{
-    AddColumnsResult, AddResult, AlterColumnsResult, DeleteResult, DropColumnsResult, MergeResult,
-    Table, UpdateResult,
+    AddColumnsResult, AddResult, AlterColumnsResult, DeleteResult, DropColumnsResult, LsmWriteSpec,
+    MergeResult, Table, UpdateResult,
 };
 
 pub mod arrow;
@@ -52,6 +52,7 @@ pub fn _lancedb(_py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<AlterColumnsResult>()?;
     m.add_class::<AddResult>()?;
     m.add_class::<MergeResult>()?;
+    m.add_class::<LsmWriteSpec>()?;
     m.add_class::<DeleteResult>()?;
     m.add_class::<DropColumnsResult>()?;
     m.add_class::<UpdateResult>()?;