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>

nodejs/src/remote.rs

@@ -145,6 +145,7 @@ impl From<ClientConfig> for lancedb::remote::ClientConfig {
             id_delimiter: config.id_delimiter,
             tls_config: config.tls_config.map(Into::into),
             header_provider: None, // the header provider is set separately later
+            mem_wal_enabled: None, // mem_wal is set per-operation in merge_insert
         }
     }
 }