feat: add mem_wal flag to merge insert for MemWAL write path

Add support for enabling MemWAL (Memory Write-Ahead Log) mode on merge insert
operations. This allows streaming writes to route through memory nodes for
high-performance buffered writes.

Changes:
- Add `mem_wal` field to MergeInsertBuilder with validation
- Add `x-lancedb-mem-wal-enabled` header for remote requests
- Add Python `mem_wal()` method to LanceMergeInsertBuilder
- Add validation to ensure only upsert pattern is supported:
  - when_matched_update_all() without filter
  - when_not_matched_insert_all()
- Throw NotSupported error for native tables
- Add mem_wal_enabled to ClientConfig for Python/Node bindings

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

python/python/lancedb/merge.py

@@ -34,6 +34,7 @@ class LanceMergeInsertBuilder(object):
         self._when_not_matched_by_source_condition = None
         self._timeout = None
         self._use_index = True
+        self._mem_wal = False
 
     def when_matched_update_all(
         self, *, where: Optional[str] = None
@@ -96,6 +97,47 @@ class LanceMergeInsertBuilder(object):
         self._use_index = use_index
         return self
 
+    def mem_wal(self, enabled: bool = True) -> LanceMergeInsertBuilder:
+        """
+        Enable MemWAL (Memory Write-Ahead Log) mode for this merge insert operation.
+
+        When enabled, the merge insert will route data through a memory node service
+        that buffers writes before flushing to storage. This is only supported for
+        remote (LanceDB Cloud) tables.
+
+        **Important:** MemWAL only supports the upsert pattern. You must use:
+        - `when_matched_update_all()` (without a filter condition)
+        - `when_not_matched_insert_all()`
+
+        MemWAL does NOT support:
+        - `when_matched_update_all(where=...)` with a filter condition
+        - `when_not_matched_by_source_delete()`
+
+        Parameters
+        ----------
+        enabled: bool
+            Whether to enable MemWAL mode. Defaults to `True`.
+
+        Raises
+        ------
+        NotImplementedError
+            If used on a native (local) table, as MemWAL is only supported for
+            remote tables.
+        ValueError
+            If the merge insert pattern is not supported by MemWAL.
+
+        Examples
+        --------
+        >>> # Correct usage with MemWAL
+        >>> table.merge_insert(["id"]) \\
+        ...     .when_matched_update_all() \\
+        ...     .when_not_matched_insert_all() \\
+        ...     .mem_wal() \\
+        ...     .execute(new_data)
+        """
+        self._mem_wal = enabled
+        return self
+
     def execute(
         self,
         new_data: DATA,

python/python/lancedb/table.py

@@ -4181,6 +4181,7 @@ class AsyncTable:
                 when_not_matched_by_source_condition=merge._when_not_matched_by_source_condition,
                 timeout=merge._timeout,
                 use_index=merge._use_index,
+                mem_wal=merge._mem_wal,
             ),
         )
 

python/src/connection.rs

@@ -506,6 +506,7 @@ pub struct PyClientConfig {
     id_delimiter: Option<String>,
     tls_config: Option<PyClientTlsConfig>,
     header_provider: Option<Py<PyAny>>,
+    mem_wal_enabled: Option<bool>,
 }
 
 #[derive(FromPyObject)]
@@ -590,6 +591,7 @@ impl From<PyClientConfig> for lancedb::remote::ClientConfig {
             id_delimiter: value.id_delimiter,
             tls_config: value.tls_config.map(Into::into),
             header_provider,
+            mem_wal_enabled: value.mem_wal_enabled,
         }
     }
 }

python/src/table.rs

@@ -710,6 +710,9 @@ impl Table {
         if let Some(use_index) = parameters.use_index {
             builder.use_index(use_index);
         }
+        if let Some(mem_wal) = parameters.mem_wal {
+            builder.mem_wal(mem_wal);
+        }
 
         future_into_py(self_.py(), async move {
             let res = builder.execute(Box::new(batches)).await.infer_error()?;
@@ -870,6 +873,7 @@ pub struct MergeInsertParams {
     when_not_matched_by_source_condition: Option<String>,
     timeout: Option<std::time::Duration>,
     use_index: Option<bool>,
+    mem_wal: Option<bool>,
 }
 
 #[pyclass]