Architecture decisions

Each record says what was decided, why, and what it costs. They are short on purpose: the code and its tests are the detail.

ADR	Decision	Since
001	Backends are plugins, found by name	0.7.0
002	Backends say what they can do; tasks are checked before they run	0.7.0
003	One order for choosing a backend; the task never names it	0.7.0
004	Your transform gets native frames; the engine gets the port	0.7.0
005	One transform for every engine: Narwhals, detected not declared	0.7.0
006	The run record is correct before it is sold: every row, any order, same on every engine	0.7.0

The rule behind all of them

The engine is a hexagon. The core (ubunye/core) holds the contract and the decisions; everything that touches the outside world (Spark, pandas, a database, a cloud) is an adapter behind a port, found through an entry point. The core never imports an engine. A test reads every import in ubunye/core, even ones inside functions, and fails if one names Spark, pandas or pyarrow.