Swift 6 release for DIFlowLayout.\n\n- Updates the package manifest to Swift 6.0.\n- Uses the versioned DIFlowLayoutEngine 1.0.0 dependency.\n- Adds Sendable conformance and cache-backed layout measurement.\n- Resolves unspecified and infinite width proposals to the single-row ideal width.\n- Adds Swift Testing coverage, CI, SPI metadata, and refreshed README documentation.