Chunked round-trip over DuckDB relations: intermittent deadlock in duckdb-python under thread churn #241

DuckDBHandle.supports_chunked = False disables the lazy (chunks=) round-trip for DuckDB relations. The blocker is upstream: a DuckDB query whose source is a Python-callback Arrow scan (any table registered through xarray-sql) intermittently deadlocks inside duckdb-python/CPython when other Python threads start or stop during execution.

Current state: chunked DuckDB round-trips fail fast instead of hanging; the eager path and every other handle operation run on one dedicated engine thread (the known-safe topology) and are unaffected. See DuckDBHandle in xarray_sql/lazyscan.py.