ddn.df.integration

The integration layer of ddn.df: cross-pillar composition contracts. Every other module proves its own layer; these unittests prove the layers COMPOSE - an inferred CSV source flowing through expressions, lazy/streamed execution, joins, windows and reshapes, with IO round-trips at the ends - producing the same schema, order, validity and float bits as the eager chain at every thread count.

A tests-only module: no public symbols, not exported from ddn.df's package.d. The suites stay under the ~3 s placement rule (longer suites live in the it subpackage).