feat: declare computed columns by SQL expression (#3937)

add_columns().computed("doubled", "x * 2") stores the expression in
field
metadata and commits the column empty; a later refresh fills it. Type
and
inputs are derived from the expression.

The declaration stays authoritative for its lifetime: writes that would
give
the column a value (append, update, merge, SQL insert), schema changes
that
would break the stored expression or reshape its output, metadata edits,
volatile expressions, declaration metadata arriving through any path but
the
validated declare call, and LSM write specs in either order against
latest
committed state are all refused. The LSM check also refuses on the
mem-wal
catch-up feature flag, which outlives unset and marks retained SSTable
rows.
Simultaneous declare/install interleavings conflict at commit via
lance's
mem-wal rule (lance#8539). Local tables only.

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python/python/tests/test_table.py

@@ -3854,3 +3854,22 @@ async def test_async_search_runs_embedding_on_dedicated_executor(
     assert all(name.startswith("lancedb-embedding") for name in captured_threads), (
         f"embedding ran off the dedicated executor: {captured_threads}"
     )
+
+
+def test_computed_column_declares_all_null(tmp_path):
+    db = lancedb.connect(tmp_path)
+    table = db.create_table("computed", [{"x": 1}, {"x": 2}])
+
+    table.add_columns(computed={"doubled": "x * 2"})
+    assert table.to_arrow()["doubled"].to_pylist() == [None, None]
+
+    # The declaration is durable field metadata.
+    field = table.schema.field("doubled")
+    assert field.metadata[b"computed_column.expression"] == b"x * 2"
+
+
+def test_computed_column_rejects_transforms_and_computed_together(tmp_path):
+    db = lancedb.connect(tmp_path)
+    table = db.create_table("computed_mixed", [{"x": 1}])
+    with pytest.raises(ValueError):
+        table.add_columns({"a": "x + 1"}, computed={"b": "x * 2"})